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Plants Alive
© 2006 G. F. Barclay
A Functional Approach
to the Study of Plant Structure
© 2006 G. F. Barclay - Plants Alive
Contents
Topics
1 Support Tissues
1
2 Vascular Tissues
4
3 Protective Tissue Layers
7
4 Secretory Cells and Tissues
10
5 Meristems
13
6 Plant Architecture
16
7 Modifications of Leaves, Stems, and Roots
19
8 Grasses
23
9 Trees and Wood
26
10 Flowers and Their Modifications
29
11 Agents of Pollination
32
12 Fruit and Seed Modifications and Dispersal
34
13 Making Biological Drawings
36
14 Making Hand Sections of Plant Material
38
15 Instructions for Using the Microscope
40
Exercises
1 Anatomy of Herbaceous and Woody Stems
43
2 Leaves and Roots
47
3 Crop Plant Anatomy and Morphology
50
4 Flowers
52
5 Fruit Structure and Classification
54
Appendices
Confusing Terms
57
Course Objectives
58
Terms to Know
59
© 2006 G. F. Barclay - Plants Alive
Copyright © 2001 and 2006 by Gregor Barclay
All rights reserved. No part of this publication may be
reproduced, stored in a retrieval system, or transmitted,
in any form or by any means, electronic, mechanical,
photocopying, recording, or otherwise, without
the prior written permission of the author.
1
© 2006 G. F. Barclay - Plants Alive
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1
Support Tissues
Higher plants use a number of different tissues to support themselves. The relative
importance of the different support tissues in a given plant depends both on its
maturity and on its environment. As the following overview discusses, various tradeoffs
occur in the kind of support tissues a plant may have, to optimize growth, adaptability,
and survival.
Young plants growing in a sheltered environment require relatively little support. Often
such plants grow under dim light conditions, such as in the shade of other plants, and
the support tissue must require a relatively small quantity of photosynthate, because
little is available. Primary walls are less costly for a plant to make than secondary walls,
and the main support tissue in such a plant is collenchyma, which is a living tissue with
primary cell walls that are thickened for support.
Collenchyma occurs in the cortex just beneath the epidermis of the stem. If the stem
has ridges, it is there that the collenchyma will be prominent. The celery stalk, which
is really a large petiole, has abundant collenchyma in the ridges which run along its
surface. The midribs of dicot leaves characteristically are strengthened with
collenchyma. Sometimes collenchyma contains chloroplasts and does double duty by
producing photosynthates as well as holding up the plant. The walls of collenchyma
cells are essentially as permeable as are those of the parenchyma cells, and so this
tissue can participate in physiological processes with the surrounding tissues.
Collenchyma occurs in different forms which can be distinguished by their pattern of
wall thickenings. All of them have primary walls that are thin in some regions and thick
in others. The cells of angular collenchyma have walls which are thicker where three
or four cells meet, forming "corners." Lamellar collenchyma has cells with walls lying
parallel to the surface of the stem that are much thicker than those lying at right angles,
giving it a layered appearance (in rare cases the walls at right angles to the surface are
thicker). Lacunar collenchyma has air spaces amongst its cells. Annular collenchyma,
a relatively rare form, has a layer of lignified wall laid down between the primary wall
and the plasmalemma.
Collenchyma provides support only when the plant is not under water stress because,
its cell walls usually contain no lignin or other hydrophobic component. Thus it does
not prevent wilting, and if the plant starts to dry out, the collenchyma loses water too
and wilts with it. Nevertheless, collenchyma is an important support tissue in the stems
and leaves of dicots. (It is rarely found in dicot roots, and it is comparatively uncommon
in monocots.)
Because collenchyma is a living tissue, it readily adapts to the support requirements of
the plant. For example, collenchyma allows a plant to bend without breaking, giving
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growing shoot tips the support they need and growing with them, in effect providing
plastic support. This plasticity is made possible by the relatively high hemicellulose
content of collenchyma cell walls compared to ordinary, primary cell walls.
More robust support is provided by sclerenchyma. Its cells have thick lignified
secondary walls, which make it both strong and waterproof. This tissue helps prevent
wilting, but it is expensive in terms of energy and metabolites for the plant to make. It
is a more permanent tissue than collenchyma and provides elastic support to maintain
the established shape of the plant.
Sclerenchyma is widely distributed in plants, occurring as a bundle cap to the outside
of the phloem in vascular bundles, and as a bundle sheath completely surrounding
vascular bundles (especially in monocots). The bundle cap physically protects the inner
tissues of the stem. The most protected tissues are the phloem and vascular cambium,
sandwiched between regions of phloem fibers and xylem. In grass leaves, the bundle
sheath may extend to the epidermis, forming a bundle sheath extension.
Sclerenchyma cells only become mature when the surrounding cells stop growing. They
are usually dead at maturity, although the lumens of the cells remain connected by pits.
Sclerenchyma cells occur in two forms: fibers, which are long (hemp fibers, which can
be up to 55 cm long, are the longest) with tapered ends, and sclereids, which are more
or less isodiametric. Brachysclereids, or stone cells, form in clumps in the flesh
(mesocarp) of the Bartlett pear (Pyrus communis var. Bartlett), giving it a characteristic
grittiness. They form a dense layer to make the endocarp ("shell") of the coconut.
Astrosclereids, which are star-shaped or branched, are found scattered through the
petioles and blades of water lily leaves (Nymphea sp.). These sclereids make the leaves
leathery and resistant to the tearing forces of waves and currents. Many seed coats
(testas), especially those of legumes, are made of a double layer of sclereids. Each layer
is one cell thick: an outer layer of oblong macrosclereids, and an inner layer of spool
or bone-shaped osteosclereids.
Some botanists classify sclerenchyma into conducting and nonconducting types. The
conducting types are made up of xylem vessel elements and tracheids, the tracheary
elements of plants. The nonconducting types are made up of fibers and sclereids.
Conducting sclerenchyma provides support as well as a transport pathway for the plant.
The fact that a tree trunk can be thought of as just a massive cylinder of sclerenchyma
is, just the same, an oversimplification. Wood (really secondary xylem) can be complex
in structure; it differs depending on the loads imposed on it in the tree. Trees produce
reaction wood if they are leaning or have heavy branches.
In angiosperm trees this appears as wider growth rings on the side opposite the lean.
This tension wood is rich in gelatinous fibers that have abundant cellulose rather than
lignin. These fibers can contract to pull up and counteract the lean. Conifers have wide
growth rings on the leaning side that contains lignin-rich compression wood, which
pushes up rather than contracts.
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The presence of reaction wood decreases the value of forest trees as timber because
boards cut from them warp more during curing than do those cut from normal wood.
Some tree species growing on a hillside with merely a 10% slope may have reaction
wood in their trunks. This factor may help prevent or slow deforestation of the
Northern Range.
Generally "hardwood trees" (angiosperms such as mora, teak, and mahogany) are rich
in fibers, making them denser, heavier, and stronger than "softwood trees”
(gymnosperms such as Caribbean pine, Pinus caribbeansis; and Peruvian pine, Araucaria
sp.), which are poor in fibers and rich in tracheids.
Ordinary, thin-walled, parenchyma cells are too often described as “packing tissue,”
simply taking up the space between other, outwardly more important tissues. But
parenchyma cells do contribute support if they are turgid. The swollen protoplast of
each cell presses outwards against the cell wall, and all of the parenchyma cells
together press out against the restraining layer of collenchyma and epidermis of the
outer stem. There is experimental evidence showing that pressure in the pith
contributes to the growth of stems.
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2
Vascular Tissues
Higher plants have two main transport systems, the xylem and the phloem, which
comprise their vascular tissue. They have somewhat different functions, but generally
arise together and are nearly always found running side by side within all organs of the
plant.
The xylem transports water and various dissolved ions from the roots upwards through
the plant. The phloem transports a solution of metabolites (mainly sugars, amino acids,
and some ions) from "sources" of production, such as fully expanded leaves, to "sinks,"
such as developing leaves, fruits, and roots. Both tissues contain long tubular cells
joined end to end which are responsible for transport, and some other associated cells.
Fundamental research on transport in plants was done by Mason and Maskell at the St.
Augustine Campus of the University of the West Indies in the 1920's and 1930's.
The phloem is predominantly a living tissue, consisting of sieve tubes, companion
cells, phloem parenchyma cells, and phloem fibers. Each sieve tube consists of a file
of sieve tube members, more commonly called sieve elements. Technically, sieve
element is the collective term for the sieve tube members, in most angiosperms and the
sieve cells in non angiosperms. But this discussion will not deal with the somewhat
different, and much less well understood sieve cell, so the term "sieve element" is used
in place of the wordy "sieve tube member."
Sieve elements are joined by thick end walls called sieve plates pierced by large,
modified plasmodesmata called sieve pores. Sieve elements contain only a small
amount of cytoplasm, which usually contains amyloplasts (starch-filled plastids) and
filamentous proteins. When mature, they lack a nucleus and tonoplast (vacuolar
membrane), but retain a plasmalemma and exhibit plasmolysis if treated with a
solution of appropriate tonicity (osmotic pressure). To this extent sieve elements are
living cells, even though they do not contain nuclei. Sieve elements have thick
primary cell walls through which pass abundant plasmodesmata, connecting them
to small companion cells that surround each sieve element.
In contrast to the rather empty sieve elements, companion cells contain large nuclei,
dense cytoplasm with abundant organelles (especially mitochondria), and many small
vacuoles. These features are related to the function of the companion cell in loading
(and unloading) metabolites into the sieve elements.
Sieve pores are lined with a special wall material called callose. This substance rapidly
proliferates to seal the pores in response to damage to the phloem such as that caused
by grazing animals. Callose formation, displacement of the filamentous proteins (which
are thought to normally line the cell wall) and release of starch grains from burst
plastids to help block sieve pores, perhaps limit sap loss from the phloem. This gives the
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phloem a special sensitivity that has made it difficult to study under the microscope.
Xylem, in contrast to the phloem, is a mostly dead tissue, and consists of vessels,
tracheids, xylem parenchyma, and fibers. The conducting cells are the vessels and
tracheids, and together these comprise the tracheary elements. These are dead at
maturity, and have thick, lignified secondary walls. Vessels consist of cylindrical
vessel elements (also called vessel members) joined together by large openings in their
end walls called simple perforation plates.
The openings are usually so large that the perforation plate appears simply as a ridge
of cell wall running around the inside of the vessel, and it is easily overlooked. More
elaborate multiple perforation plates replace the simple ones in vessels at intervals
of some centimeters. These are open lattice works of primary cell wall material that
form distinct end walls. They are more common in less advanced plants.
Secondary wall, consisting of cellulose that is rich in the hydrophobic substance lignin,
is laid down as a distinct layer inside the vessel. It begins to form when the cell is
young, full of cytoplasm, and still living, and takes different forms because vessels can
elongate by different amounts during development. Secondary wall deposition and cell
elongation are accompanied by other changes in the developing vessels, including
perforation of the end walls, disappearance of the nucleus, and loss of the cytoplasm.
Lignin deposition stops when the cell dies, and so xylem tissue characteristically
contains vessels with different states of lignin deposition. These range from annular,
helical, scalariform, reticulate, to pitted, reflecting progressively more lignin
deposition. Types intermediate between these are recognized in some plants, while in
others fewer forms occur, and in a few species only one type has been found.
Although transport can occur freely through the perforation plates of vessels, movement
also takes place laterally, among adjacent vessels, and with adjoining xylem
parenchyma cells. This happens across areas of primary wall where no secondary wall
has been deposited. Vessels with annular thickenings provide the greatest area for
lateral transport, with less area becoming available as lignification increases.
Lateral transport in pitted vessels is restricted to structures called pits. These occur as
two main types, simple pits and bordered pits. Simple pits are, as the name implies,
simply areas of bare primary wall in vessels otherwise covered with lignin. In bordered
pits, a ring of secondary wall surrounding the exposed primary wall, arches upward like
a blister, creating a chamber. The exposed primary wall is modified into a thickened,
lens-shaped torus suspended across the middle of the pit chamber by a porous ring of
cellulose fibrils called a margo. Water is free to flow between adjacent vessels through
the margo so long as the torus remains in the middle of the chamber.
A large pressure difference between the vessels, such as that caused by an air embolism
in one vessel, will displace the torus against the inside of the overarched secondary
wall, preventing passage of water. Thus, the bordered pit is a pressure sensitive valve,
controlling water flow in the xylem.
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The other type of tracheary element is the tracheid, which has tapered ends and no
perforation plates. It is connected to adjoining cells only by pits, and has secondary wall
thickenings that can be of different types, like in vessels. While tracheids are found in
both gymnosperms and angiosperms, only the most advanced gymnosperms contain
vessels. Ten genera of the lowermost taxonomic groups of dicots have only tracheids,
not vessels.
Tracheids permit conifers to dominate northern forests because they allow these trees
to tolerate freezing. When water freezes, dissolved air comes of solution and forms
bubbles. When the ice melts, the bubbles remain and break the columns of water
essential for transport in xylem vessels, rendering them non functional. But air bubbles
that form in tracheids are trapped in the pointed ends of the cells. Transport continues
across the side walls between adjacent tracheids through bordered pits. Although
tracheids transport water less efficiently than vessels, the wood of conifers may consist
almost entirely of these cells, making up for their lack of efficiency.
Despite the survival advantage that tracheids have given the gymnosperms, it is clear
that tracheids are less "advanced" than vessels, and that tracheids evolved from fibers.
Some extant (present day) plants have fiber-tracheids, intermediary in structure
between the two types of cell. More attention has been paid to the phylogenetic
development of xylem than to any other tissue (phylogeny is the history of a taxonomic
group from an evolutionary viewpoint).
Tracheary elements that are more extensively covered with lignin provide the plant
with more drought resistance, because they lose water less readily. More lignin also
means more mechanical support for the plant, but as lignin becomes thicker, the lumens
of the tracheary elements become narrower, so less transport can occur.
The influence of vessel diameter on flow is dramatic. Volume flow rate is proportional
to the square of the radius of the tube, and the pressure required to force water through
a tube is proportional to the fourth power of the radius. Also, when a plant devotes
more energy and metabolites to bolstering its existing xylem in this way, less of these
are available for other purposes, such as making new leaves or longer stems.
Compromises in structure occur here as a natural part of plant development.
A third transport system occurs in plants, consisting of laticifers. These are tenuous,
thin-walled cells that are made either of long files of cells whose end walls have
degraded, or simply individual cells that arise in the embryo and grow in length with
the plant. Both types can traverse roots, stems, and leaves, and contain a wide range
of substances including latex, alkaloids, and terpenes. Many of their contents apparently
have a rôle to play in protecting the plant from pathogens and herbivores.
Laticifers are, strictly speaking, examples of secretory cells rather than vascular cells,
but fluids do move in them through the plant. Scarcely any functional analyses have
been done on laticifers, unlike the xylem and phloem, which have received intense
scrutiny by physiologists. The idea that laticifers are the plant's lymph system, and the
xylem and phloem its arteries and veins, is old and simplistic.
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3
Protective Tissue Layers
All plants are wrapped in protective layers of cells, and other such layers occur inside
them. An epidermis entirely covers herbaceous plants: leaves, stems, and roots all have
an epidermis, which may be defined as the outermost cell layer of the primary plant
body. The epidermis arises in the embryo from the protoderm, and new epidermal cells
form in meristematic areas on shoot and root apices to keep the growing plant covered.
The epidermis in many plants is just one cell thick (uniseriate), although a few plants
have a multiple (multiseriate) epidermis many layers thick. Epidermal cells may be
long lived (10-20 y) and are modified in diverse ways.
The epidermis, which forms an interface between the plant and its environment, is
coated with cutin, a complex fatty substance that forms the cuticle. Waxes embedded
in the cuticle render it very impermeable to water, and is indigestible by pathogens,
hence epidermis keeps water in the plant and pathogens out. It is usually covered with
a surface layer of epicuticular wax, which enhances these qualities. The cuticle is clear,
as are most of the epidermal cells it covers, allowing light to reach the photosynthetic
tissues beneath. Also, the cuticle selectively protects the plant from ultraviolet radiation
from the sun, which is mutagenic and potentially harmful to the plant.
The epidermis must allow gas exchange between the air surrounding the plant and the
tissues inside it, otherwise photosynthesis and respiration cannot occur. Therefore, it
has special structures called stomata. Each stoma (also called stomate) consists of a
pair of guard cells that can change shape to create a stomatal pore. Guard cells have
large nuclei and numerous chloroplasts (usually they are the only epidermal cells to
have chloroplasts), and seem to be functionally isolated from the surrounding
epidermal cells. In most dicots, guard cells are kidney shaped, and the inner walls,
separating the guard cells, are thicker than outer walls of the pair. Some of the cellulose
microfibrils in the cell walls are arranged in hoops around the guard cells, and the ends
of the cells are attached to each other.
When the guard cells absorb water, they expand, but the restraining hoops of cellulose
force them to get longer rather than wider. The thicker inside walls cause the guard
cells to bend apart, creating a pore for gas exchange. In most monocots, the guard cells
are dumbbell shaped. The walls of the middle part of the cells are relatively thick, and
those of the bulbous ends are relatively thin. When these guard cells absorb water, their
bulbous ends expand, again creating a pore for gas exchange.
Trichomes arise from epidermal cells that extend outward, typically dividing repeatedly
to form a single file of cells. They are more common on younger parts of the plant.
Some trichomes have a more complex multicellular origin, arising from one or more cell
layers. Trichomes have a number of functions, including shading the plant surface from
excessive light, reducing air movement (and thus excessive desiccation), and protecting
the plant from insects and herbivores. The latter function may be passive (simply
© 2006 G. F. Barclay - Plants Alive
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blocking access to the plant surface) or active, through secretion of toxins.
Trichomes on leaves and stems of the stinging nettle have brittle silica tips that readily
break off to inject grazing animals or passing hikers with irritating histamines. The most
highly modified trichomes are perhaps those on the leaves of the insectivorous sundew
(Drosera sp.), which go beyond protection to predation. These excrete a sticky nectar
to attract and hold insects, then bend over in concert with leaf folding to trap them,
excrete digestive enzymes, and finally absorb nutrients from their prey.
Root epidermis is far less complex than shoot epidermis, and it has different functions.
The shoot epidermis must protect the plant from desiccation, while the root epidermis
must allow the plant to extract water from the soil. Therefore, the root epidermis has
little wax in its cuticle to interfere with water uptake, and it often secretes mucigel,
especially from the root cap. Mucigel is a hydrophilic carbohydrate that absorbs water
to help lubricate the passage of the root through the soil and to mobilize ions from it.
Mucigel also fosters growth of soil fungi that in turn provide nutrients to the root.
Root hairs are short-lived (one or two days), fragile, outgrowths of epidermal cells that
greatly increase the volume of soil that a plant can mine for nutrients. Roots of plants
grown in solution culture tend to lack root hairs.
Plants in secondary growth can replace their epidermis with a more substantial tissue
called periderm. This usually arises just beneath the epidermis, which then dies and
eventually sloughs off. It may form in other areas of the cortex, in the epidermis itself,
or in the vascular tissue.
Periderm consists of layers of three types of cell: a middle layer of phellogen (cork
cambium, a lateral meristem) that produces an outer layer of phellem (cork cells) and
an inner layer of phelloderm (which contributes to the cortex). Cork cells are dead at
maturity and their walls have layers (as many as 10-20) of suberin and sometimes
lignin, giving them great resilience to desiccation and insect or pathogen attack.
Periderm also develops in roots if these develop secondary growth, but here it usually
arises from the pericycle, which is just outside the phloem.
Gaps occur in the phellem called lenticels, which arise mostly under stomata (which
they replace) in the epidermis and persist as blisters in tree bark. They have loosely
arranged cells that allow gas exchange with the underlying tissues that the periderm
so well protects. Because lenticels are passive pores (unlike the actively controlled
stomata), they provide a constantly open route for infection and herbivory.
The term "bark" generally means all of the tissues external to the vascular cambium.
These comprise the secondary phloem, phloem fibers, cortex (which may contain
collenchyma), and the periderm layers. Bark shows many patterns because periderm
forms at different rates at different places on the stem. Also, the cork breaks apart as
the stem expands in circumference. Cork cambia may live only for weeks or survive for
years. Either way, new cambia arise under other ones, pushing the old periderms out.
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In most plants the cork cambium, unlike the vascular cambium, does not live for the life
of the plant. But, Quercus suber (the cork oak)has only one layer of cork cambium and
it remains alive for the life of the tree. This allows a uniform layer of cork to be
continuously produced. Because of seasonal variation in growth rate, wine bottle corks
show growth rings.
Subdermal protective cell layers occur in roots, stems, and sometimes leaves. The most
prominent such layer is the endodermis, commonly found in roots. The endodermis
forms the innermost cell layer of the cortex, separating it from the stele (vascular
cylinder). The endodermal cell wall contains suberin, the same substance found in cork
cells. However, here it is usually restricted to a narrow, thin strip called a Casparian
band that runs around the middle of each cell in its radial and tangential walls (the
walls that touch other endodermal cells).
Despite its small size the Casparian band effectively prevents uncontrolled entry, of
water and ions absorbed from the soil, into the stele through the apoplast (the "free
space" of the cell walls and intercellular spaces). This could swamp the vascular tissue
with unneeded material. Instead, the Casparian strip directs transport through the
cytoplasm of the endodermal cell, which subjects it to physiological influence, allowing
selective uptake by the plant.
The endodermal cells of many plants are extensively lignified, forming characteristically
U-shaped ("phi") thickenings. Although an endodermis is primarily a feature of the root,
sometimes it does occur in the stem. Here, it usually lacks a Casparian band or phi
thickening, and special staining methods are needed to make it visible. Stem
endodermis is sometimes referred to as a starch sheath, but since the starch itself may
be absent, the term is not useful. Some aquatic plants do have an obvious endodermis
in their stems, and in their leaves as well.
The roots of some plants, especially monocots, have an exodermis directly beneath
their epidermis. The exodermis (also called hypodermis)the outermost cell layer of the
cortex and it is frequently lignified. It can have a Casparian band and may occur in
stems. Perhaps, it functions like a second endodermis.
Vascular bundles, especially in monocots, are often surrounded by a bundle sheath of
sclerenchyma fibers that isolates transported material within the pathway, in effect
protecting it from premature loss. The fruit wall (pericarp, derived from the ovary
wall) and seed coat (testa, derived from the integuments in the ovary) are important
protective tissues.
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4
Secretory Cells and Tissues
Secretion is an important and diverse (but often overlooked) function of plants. All
plant cells are, at a fundamental level, secretory. The cell wall is a secretion of the
protoplast, and the wall itself secretes (among other things): cuticle and waxes that
cover the epidermis, lignin that is secreted to the inside of the primary cell wall during
secondary wall formation, and pectin (as calcium or sodium pectate) that forms the
middle lamella, the "glue" bonding cells together. Various organelles, such as
dictyosomes and endoplasmic reticulum, are secretory, in that they produce cellular
components, so the definition of secretion, if taken to an extreme, can lose its
usefulness.
Secretion has two main functions: aiding metabolism (for example, by removing excess
salts or water, and isolating or removing toxins from the plant), and facilitating the
plant's interaction with its environment (a function performed by nectaries, stinging
trichomes, poison cells, digestive glands, scent producing glands, and so on.)
Secretions may be classified as being external or internal.
External secretions
Nectar is composed mostly of sugars and is produced by floral and extrafloral
nectaries to attract pollinators, which may exist exclusively on it. Nectaries can be
single celled or multicellular. The nectar is secreted between the wall and cuticle, where
it accumulates and eventually bursts out. Floral nectaries are common and occur on
many flowers, while extrafloral nectaries are relatively uncommon and, as the name
suggests, they are found elsewhere on the plant. The sugars produced by the extrafloral
nectaries of Acacia attract ants, which then colonize the tree and defend it against
herbivores.
Water is excreted in an almost pure state at leaf margins by hydathodes, which occur
in more than 350 genera in 115 families. High soil moisture and cool/humid air
promote guttation of water, and a single Colocasia (dasheen) leaf may gutate 100 ml
of water in one night. The purpose of this phenomenon becomes clear if the presence
of hydathodes in hydrophytes is considered. While such structures ought to be out of
place in aquatic plants, they are thought to pull water out of the plant, replacing
transpiration pull. Similarly, hydathodes in terrestrial plants may pull water through the
xylem when conditions for transpiration are unfavorable.
Salt is produced by salt glands in halophytes. These remove salts that the root
endodermis cannot exclude, and the crystals can coat leaves to discourage herbivores.
Coastal mangrove trees may be inundated with salt, which they must continuously
excrete to survive.
Odours and perfumes are produced by osmophores. The substances they produce are
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often volatile terpenes, and function to attract pollinators. Osmophores in the Aroids
may give off amines/ammonia compounds, imitating rotten meat smell to attract
pollinating flies.
Digestive enzymes are exuded by the highly modified leaves of insectivorous plants
such as Drosera (sundew).
Adhesives are exuded from glands covering attachment organs in climbers (e.g.,
Philodendron sp.) and parasitic plants (birdvine).
Internal secretions
Resins occur in long cavities or ducts. Resins are usually terpenes, commonly found in
conifers.
Mucilage, a carbohydrate with a slimy consistency and a high water content, is found
in dessert succulents, seed surfaces (to attract water and aid germination), root caps,
and insect traps. Opuntia mucilage retains water even in 75% alcohol, demonstrating
its ability to help this cactus store water.
Gums form in certain kinds of wood from cell wall modification or breakdown.
Oils, ranging from low molecular weight and aromatic (volatile) to high molecular
weight and waxy occur in many plants. They are usually produced in cavities in the
plant for example in the rind (pericarp) of citrus fruit, but they can be released on the
plant surface where they evaporate, for example on thepetals of the fragrant ylang
ylang.
Toxins Mustard oil is produced when the enzyme mryosinase in the vacuole of a
mryosin cell mixes with a thioglucoside substrate in the cytoplasm to form toxic
isothiocyanates. The two components are harmless so long as they are kept apart, and
only become toxic when a herbivore chews on the plant and inadvertently mixes them.
Mryosin cells, common in the mustard family, must occur in all edible parts to deter
herbivores. Acetocholine, histamine, and other toxins are stored in the vacuoles of
stinging trichomes. The trichome tip is brittle silica or calcium oxalate that breaks to
impale herbivores and release the toxin. They are found in 4 families in 4 orders of
plants, an example of extreme convergent evolution.
Latex is secreted and transported by laticifers. It forms the milk of milkweeds, poppies,
Euphorbiaceae, etc. Laticifers occur in about 12,500 species in 900 genera of plants,
and have been found to contain carbohydrates, salts, alkaloids, sterols, lipids, tannins,
mucilage, camphor, rubber, protein, vitamins, starch grains, and even living protozoa.
The functions of some of these contents are obscure, but latex does make plants
unpalatable, if not poisonous, to herbivores. Many unripe fruits contain latex to deter
frugivores until the seeds within are mature and ready to be dispersed.
Simple laticifers (nonarticulated laticifers) are multinucleate (sometimes containing
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thousands of nuclei), individual cells that arise in the embryo and grow with the plant.
They traverse roots, stems, and leaves, and can branch. They may show invasive
growth, with the latex of some laticifers found to contain pectinase to soften the middle
lamella. Their multinucleate nature is a feature shared with coencytes, which are single
celled, lower plants that are also multinucleate. This similarity, and their ability to grow
invasively through other tissues, suggests that laticifers are in effect living separately
within the host plant.
Compound laticifers (articulated laticifers) are fundamentally different from simple
laticifers. Each is a row or file of single laticifers joined end to end. They begin as
ordinary cells that join up. In onion the cells are connected only by plasmodesmata,
while in Musa (banana) the entire end wall disappears, like in xylem vessels.
Compound laticifers may branch to form three dimensional networks. Hevea braziliansis
laticifers have lutoids 1-5 :m in diameter full of rubber. Laticifers are thin-walled, hard
to trace through the tissues they traverse, and difficult to prepare for microscopy. Little
functional analysis has been done to try to get a basic understanding of latex formation
and transport as an aid to improving latex yield.
Gases, which are not just air, are found in intercellular spaces. They have a high
concentration of CO2, O2, or ethylene, and are a product of cell metabolism. In
hydrophytes the gas in the aerenchyma tissue aids buoyancy. Ethylene production
within the pith leads to degradation of the middle lamella, which causes the cells to
separate from each other, producing a hollow pith.
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5
Meristems
Growth patterns, both anatomical and morphological, are determined by the activity
of meristems. These are groups of cells specialized for the production of new cells and
they are capable of dividing more or less indefinitely.
Meristematic activity becomes apparent in the embryo, very early in development. In
some plants, when the ovule reaches the16 cell stage of development, there is already
an outer layer consisting of 8 of these cells. This layer is the first discernable
protoderm, which is the primary meristem giving rise to the epidermis of the plant.
There are only two other fundamental primary meristems in the embryo, the ground
meristem, which produces cortex and pith, and the procambium, which produces
primary xylem and phloem.
To help understand the function of meristems in plants, it is useful to compare plant
development with animal development.
Plants exhibit a fundamental difference in development from animals. Animals establish
as embryos a pattern of form that simply gets larger as growth occurs. Broadly
speaking, animals do not add new parts as they grow. Although some changes occur in
proportion, in general animals increase in size by diffuse growth, with all parts
growing at once at more or less the same rate. Their cells only divide a few times,
between embryo and adult, allowing little chance for mutation.
As a very simplified example, a series of synchronized, repeated equal cell divisions,
each resulting in a doubling of mass, will allow a 4 kg "infant" to grow to a 64 kg "adult"
with just four divisions of its cells (4 kg x 2 = 8, 8 kg x 2 = 16, 16 kg x 2 = 32, 32 kg
x 2 = 64 kg). Because animals use this growth plan, all the individuals of any species
are so similar that they are essentially identical in form. Most animals reach a fixed
adult size and do not live beyond a fixed maximum age. They are not forced to
physically adapt to local conditions because they are free to move around.
In contrast, most plants never stop growing and are thus indeterminate. They have no
fixed size or age, and many annuals become perennials if they are protected from
environmental changes in a greenhouse. (Monocarps, which flower once and then die,
are an exception. Such plants exhibit determinate growth.) Because they are rooted to
one spot, plants must adapt to changing conditions in order to survive. Also, most
plants become damaged or lose parts as they grow, so they must be able to rejuvenate
these, and continue the cycle of producing flowers and setting seed.
The need for such continual renewal has led plants to have localized growth, and this
growth occurs in meristems. Meristems are sources of undifferentiated, mitotically
young and genetically sound, cells. They allow plants to have undifferentiated tissues,
growing tissues, and mature tissues all at the same time. In other words, all plants,
14
© 2006 G. F. Barclay - Plants Alive
except seedlings, have constantly juvenile, meristematic cells, differentiating derivatives
of these, and fully differentiated, adult cells. Thus, a plant will have fully developed,
functioning organs, while still growing.
Plants are prone to mutation because they are potentially long-lived and also because
they are subjected to ionizing UV light from the sun. Mutations are most likely to occur
during cell division, so the fewer cell divisions plants need for growth, the better.
Meristems let the plant avoid repetitive cell divisions, reducing the possibility for
mutation. But because plants have growth localized in meristematic areas, these are at
risk of accelerating mitotic mutations if all they do is create cell lines (cells of the same
type) by endless cell division. Plants avoid cell lines by having multistep meristems.
Plants, because they are potentially long-lived and also because they are subjected to
ionizing UV light from the sun, are more prone to mutation than are animals. Mutations
are most likely to occur during cell division, so the fewer divisions plants need for
growth, the better. Meristems let the plant avoid repetitive cell divisions, reducing the
possibility for mutation. But because plants have growth centered in meristematic areas,
these are at risk of accelerating mitotic mutations if all they do is create cell lines (cells
of the same type) by endless cell division. Plants avoid cell lines by having multistep
meristems.
This multistep meristem produces secondary vascular tissue:
Vascular cambium !
[! Fusiform initials [! Phloem mother cells
[
[! Xylem mother cells
[
[
[! Ray initials
[! Xylem rays
[! Phloem rays
This delegation of roles makes cell lines much shorter. Endlessly repeating cell divisions
are not eliminated, but they tend to occur as the end product of the meristem, that is,
identical cells that do not divide further.
A good example here is the phellem (cork) produced by cork cambium. Cork cells are
produced in great numbers and they are dead when functional. They do not divide
further, and it does not matter if some of these cells are mutated because their genes
are not passed on. Reduction in mutation enhances the potential for plants to grow
indefinitely. While “Eternal God” a coast redwood (Sequoia sempervirens) in California
has lived for more than twelve millennia, it is still young when compared to clonally
reproducing specimens of King’s Holly (Lomatia tasmanica) in the wilderness of
Tasmania that are 43,400 years old. Meristematic activity in these and other plants of
great age have allowed them to achieve states of near immortality.
Meristems establish growth patterns in plants, and they are responsible for the
wonderful geometric forms which plants take. At the same time, they allow growth
plasticity so that the plant can adapt to its environment.
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Because they are rooted to one spot, plants must adapt to changing conditions in order
to survive. They must be able to rejuvenate parts that are damaged or lost as they grow,
and continue the cycle of producing flowers and setting seed. For these reasons plants
are able to continually renew themselves through localized growth, a process that
occurs in meristems. These are sources of undifferentiated, genetically sound, cells.
Determinate meristems are designed to produce structures of a certain size, such as
leaves and flowers. The great similarity of the leaves on a tree results from the
effectiveness of determinate meristematic activity in creating copies of structures.
Indeterminate meristems are never ending sources of new cells, allowing increase in
length (apices) or girth (cambia).
Meristematic activity arises early in embryogenesis. In some plants, when the ovule
reaches the sixteen-cell stage of development, there is already an outer layer consisting
of eight of these cells. This layer is the first discernable protoderm, which is the primary
meristem giving rise to the epidermis of the plant. Two other fundamental primary
meristems arise in the embryo, the ground meristem, which produces cortex and pith,
and the procambium, which produces primary vascular tissues. In shoot and root tips,
apical meristems add length to the plant, and axillary buds give rise to branches.
Intercalary meristems, common in grasses, are found at the nodes of stems (where
leaves arise) and in the basal regions of leaves, and cause these organs to elongate. All
of these are primary meristems, which establish the pattern of primary growth in plants.
Stems and roots add girth through the activity of vascular cambium and cork cambium,
lateral meristems that arise in secondary growth, a process common in dicots. Many
monocots have primary meristems alone, and lack true secondary growth. Cambium is
in essence an intercalary meristem because it lies between its derivatives. Vascular
cambium normally creates xylem to the inside and phloem to the outside of stems and
roots, (just as cork cambium lies between its derivatives). But, primary intercalary
meristems in time stop producing new cells and disappear, whereas cambium is
essentially indeterminate in its activity. The activity of the vascular cambium is
complex. It produces more xylem than phloem, and thus it expands in circumference
and it must add new cells by radial divisions to maintain the integrity of the cambial
cylinder.
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6
Plant Architecture
Plants adopt a bewildering variety of form, designed primarily to optimize survival in
various habitats so that they proliferate and ultimately perpetuate the species through
the production of seed. Plants are inherently branched: a branching root system mines
the soil and anchors the plant, a branched stem holds leaves in an efficient way to
intercept sunlight, and clusters of flowers attract pollinators to fertilize them.
Leaf form and arrangement
The leaves on a plant usually have a fixed shape: all its leaves are essentially identical.
(Note: the cotyledons, which begin as storage leaves and later become photosynthetic,
often differ from the rest of the leaves on the plant.) Some plants exhibit heteroblasty
(also termed heterophylly) and have more than one shape of leaf (usually two). The
aquatic monocot arrowhead (Sagittarius) produces long strap like leaves underwater
and characteristic arrowhead shaped leaves above water. The narrow submerged leaves
allow water to flow around them, and the broad aerial leaves, which would be a
liability in a strong water current, are designed to intercept light. Leaf margins are of
many types: entire, dentate, divided, lacerate, serrate, and so on.
A simple leaf has one undivided surface, while on a compound leaf the lamina is
divided up into a number of leaflets (pinnae). Leaves may be palmately compound,
with the leaflets attached to the end of the petiole, or pinnately compound, with the
leaflets attached to the sides of the petiole (which then becomes a rachis). If the rachis
is unbranched, it is unipinnate, with secondary branches, bipinnate, and with tertiary
branches, tripinnate. These terms form part of the extensive terminology used in
taxonomy for describing leaf appearance.
It is advantageous for a plant to have compound leaves rather than simple leaves. A
compound leaf flutters more easily in a breeze, facilitating cooling and at the same time
allowing it to capture CO2 from the air surrounding it. The diffusion of gases is not
great enough to replace those that are absorbed through the stomatal pores. Thus, the
more freely a leaf can move about, the better. Also, fluttering makes it more difficult
for fungus spores and pests to establish themselves on a compound leaf.
The sequence of leaves on a stem, or phyllotaxy, is also usually fixed, and affects how
much sunlight each leaf can intercept without shading its neighbours. Monocots have
one leaf per node and dicots have one or more leaves per node. The fundamental
terminology is:
A. One leaf per node (alternate):
[Monostichous (very rare). All leaves on one side of stem, forming one row seen from
above.]
Distichous. (A diagnostic feature of grasses.) Leaves are all arranged in two rows seen
from above, usually with 180° between rows.
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[Tristichous (rare outside the Cyperaceae). Three rows of leaves with 120° between
them.]
Spiral. (very common) Applies if three or more longitudinal rows are present, e.g., 5
or 8. Successive leaves in the spiral are separated by an angle that in most plants is
137.5°. This same Fibonacci angle also appears in flowers (sunflower) and fruit
(pineapple), and can be demonstrated in many species of plants in many families.
B. Two leaves per node (opposite):
Two leaves 180° apart at each node forming two rows.
Opposite decussate. Successive pairs oriented 90° to each other, forming four rows.
[Spiral decussate (bijugate). Successive pairs are less than 90° apart, producing a
double spiral.]
C. Three or more leaves per node (whorled):
A fixed or variable number of leaves arises at each node. Leaves in successive whorls
may or may not form discrete rows when seen from above.
Stem branching
The pattern of stem branching, can be fixed, like leaf arrangement, but it is more
commonly so in the lower plants than the higher ones. Branching is a complex topic,
but it is convenient to identify four basic patterns:
Monopodial: The shoot apical meristem remains rhythmically active throughout the
life of the plant. The axillary shoots are secondary and are regulated by the apex of the
main shoot (most conifers, e.g., Araucaria).
Sympodial: The shoot apex becomes reproductive or aborts. Axillary shoots grow
outward, turn upward, and produce clusters of leaves. One shoot grows upward and
becomes the main stem. Its shoot apex becomes reproductive or aborts, and so on. The
characteristic pagoda shape of the seaside almond (Terminalia catapa) results from this
growth habit.
Dichasial: A type of sympodial branching in which the terminal bud gives rise to two
axillary buds on opposite sides of the stem. These grow at a similar rate and then
branch again, resulting in a repeatedly forked appearance. Examples include pink poui
(Tabebuia pentaphylla), frangipani, and mango.
Adventitious: Although shoots usually arise from apical or axillary buds, they may
appear endogenously from any organ. The sweet potato (Ipomea batatas) owes its
scrambling habit to new stems arising from the roots, creating a disorganized
appearance to the plant. In the maternity plant (Bryophyllum pinnatum) new plantlets,
with leaves and roots, arise along the margins of the leaves of the “mother” plant.
Endogenous shoots on trunk and branches of the cocoa tree (Theobroma cacao) give rise
to pods, and in the cannonball tree (Couroupita guianensis) shoots that produce flowers
and fruits ring the trunk a few meters off the ground, both forms of caulifory.
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Floral branching (inflorescences)
Plants may have solitary flowers that are borne singly in the axil of a leaf, as in
Hibiscus, or possess branched systems of flowers forming definate patterns called
inflorescences. A great variety of these exist, but in general they can be divided into
two types. The main axis of a racemose inflorescence is not terminated by a flower,
so it is capable of indefinite growth. It displays monopodial branching and is
indeterminate. A cymose inflorescence is terminated by a flower on the main axis,
and is therefore capable of limited or determinate growth. Additional flowers must
arise from sympodial branching.
Root branching
Root branching is poorly understood. Roots grow out of sight and are inherently
difficult to study. Although some general types of root systems have been identified,
root growth is extremely plastic, and is strongly governed by soil moisture, fertility,
composition, and homogeneity. Root systems never seem to approach the geometric
patterns that stem systems can exhibit.
Roots lack axillary buds, flowers/fruits, leaves, and any real apical dominance. Under
the influence of gravitropic factors, the primary root from a seedling normally
penetrates downwards through the soil, giving rise to lateral secondary roots (often
roughly in rows, usually 4). New roots arise endogenously from the pericycle of the
main root. (These lateral roots may exist as primordia in the embryo before
germination, where they are called seminal roots.)
Lateral roots are anatomically identical to the primary root and produce tertiary roots.
Their arrangement is less well ordered than the laterals. The tertiaries may themselves
be branched, and the branching process may go on, almost indefinitely. Roots are very
indeterminate, and continue growing in patches, seeking out regions of soil rich in
nutrients or water, so that any kind of recognizable architecture is easily lost.
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7
Modifications of Leaves, Stems, and Roots
Although plants follow basic patterns of growth, adaptation to the environment
(plasticity), and modifications to perform certain functions or enhance survivability,
together produce great diversity in plant form (morphology). Modification of organs
can produce quite dramatic structures.
Leaves
The primary function of most leaves is, of course, photosynthesis, but some leaves are
modified for other purposes, and their photosynthetic role is diminished or dispensed
with.
The spines on many cacti, members of the family Euphorbiaceae, and many other
plants, are really leaves that consist entirely of compact bundles of sclerenchyma fibers.
They contain no air spaces, no parenchyma, just fibers. A spine grows from its base
located in the plant, and most of the exposed portion is dead. It functions like a sturdy
version of a trichome, defending the plant against herbivores. Spines don't
photosynthesize, so this function is often done by the stems on which they form. Spines
should not be confused with “reduced” leaves. They can, if densely packed, provide a
shield against intense sunlight and the drying effect of wind.
Thorns are, to some botanists, simply spines that may contain vascular tissue and are
modified stems, not leaves. Both arise from axillary buds and provide protection from
herbivores. Other botanists avoid the term thorn altogether.
Tendrils are leaves that lack a blade, like spines, but photosynthesize, never stop
growing, and provide support to the plant. They are touch sensitive, not light sensitive,
and coil around things, especially stems of other plants, for support. Coiling results from
growth occurring faster on the side of the tendril not in contact with the support than
the side in contact. The term for such contact or force mediated growth is
thigmomorphogenesis. The coiling response can occur along the length of the tendril
even if just the tip touches the support. This effectively shortens the tendril, drawing
the plant closer to its support and facilitating upright growth.
Tendrils are commonly found in the Cucurbitaceae (cucumber, melon), Passiflora
(passion fruit) and the Convolvulaceae. Some texts note that both spines and tendrils
can arise from axillary buds, and may be stem modifications. Indeed, spines arise from
adventitious roots on a species of mangrove tree, and some tendrils even arise from
inflorescences. But it is the result that is important, not the origin.
Sclerophyllous leaves are a xerophytic form of leaf, designed to withstand a desert-like
environment. Like spines, they have abundant fibers but retain photosynthetic ability.
Such leaves are expensive for the plant to make and so represent an investment for it,
and tend to be long-lived. Ordinary leaves, by contrast, are cheap and mass produced
© 2006 G. F. Barclay - Plants Alive
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by many plants, and are quite expendable. Sclerophyllous leaves are especially common
among the monocots, for example, Agave sisalana. The fibers that make leaves of this
plant so durable are used to make sisal rope.
Succulent leaves are another xerophytic form, with a small surface to volume ratio,
few air spaces, and a thick cuticle. The mesophyll is isolated from the leaf surface by
cells with large vacuoles. These act as a heat filter against intense sunlight. These leaves
often contain mucilage, which binds water so it can be stored in the leaf. The vascular
tissue is "conserved." There is not much of it because little transport occurs in these
leaves. Aloe vera is a good example. Both succulent and sclerophyllous leaves have a
smaller surface to volume ratio, reducing transpiration losses.
Bud scales (cataphylls) are tough, sessile (lacking a petiole), leaves (actually stipules)
that are nonphotosynthetic and which protect apical or axillary buds, during dormancy,
against herbivores and drying winds. Although mostly found on temperate trees, in the
tropics they are found on the rubber tree (Hevea brasiliensis), magnolia, and mahogany.
They commonly form cork cambium that makes a thin bark, the only type of leaf to do
so.
Trap leaves are found on insectivorous plants, which can grow in nitrogen poor soil by
getting their nitrogen from insects that they trap and digest with their highly modified
leaves. The sundew trap leaf is described in Chapter 3 of this book.
Another type of trap leaf occurs on the pitcher plant (Nepenthes, Sarracenia,
Darlingtonia). The pitcher is a highly modified leaf lamina that is typically red brown
(to look like carrion), and lined with glands that release digestive enzymes. It has such
features as a lid to exclude rain, and downward pointing trichomes and abundant flaky
wax to prevent escape of insects lured into it.
In Nepenthes the petiole of the trap is elongate, wide, and flat, like a normal lamina,
forming a phyllode. This structure compensates for the reduced photosynthetic role of
the pitcher. Phyllodes also occur on certain species of Acacia that have reduced leaf
blades. Other, more elaborate kinds of trap leaves occur on the Venus flytrap (Dionaea)
and the aquatic bladderwort (Utricularia).
Stems
Stems serve other functions besides or instead of support. Some are modified for
storage. Bulbs consist of flattened, short stems with thick, fleshy nonphotosynthetic
storage leaves (onion, Allium cepa, garlic, A. sativa). Corms are vertical, enlarged fleshy
stem bases with scale leaves (dasheen, Colocasia esculenta; nutgrass, Cyperus sp.), while
rhizomes are fleshy horizontal stems, also with scale leaves (arrowroot, Maranta
arundinacea; ginger, Zingiber officinale. Stem tubers are thick storage stems, usually
horizontal (yam, Dioscorea sp.; Irish potato, Solanum tuberosum)
All of these are both underground storage stems, and organs of perinnation, which
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enable plants to survive during periods of drought or cold, storing food reserves away
from most herbivores, in a relatively stable environment, until suitable growing
conditions return. The sugar cane stem (Saccharum) is an aerial stem tuber. All of
these storage stems are propagative, and designed to produce new shoots and roots.
Rhizomes are particularly good at allowing plants to spread underground. Stolons
allow plants to spread above ground. These are indeterminate propagative stems with
long, thin internodes that readily root at the tip to form new plants if growing
conditions are favorable (Paspalum sp., saxifrage, spider plant). They have no storage
or support function.
Although many stems in primary growth photosynthesize to augment their leaves, some
stems are more specifically designed to replace leaves. The cladode is a fair example
of a photosynthetic stem. It is flattened to look vaguely leaflike (as in prickly pear
cactus, Opuntia), but it is swollen for water storage (succulent), too. The conifer-like
needles found on the she oak (Casaurina) are better examples of photosynthetic stems.
They are not enlarged for storing anything, and they have tiny, vestigial leaves on them.
The best local example of a photosynthetic stem occurs on Rhipsalis, the dichotomously
branching epiphytic cactus that hangs from the trees on campus. Its stems are obviously
no good for support, and they are not specially modified for storage. But the stems are
the only real photosynthetic organ of this plant because its leaves are useless
microscopic bumps.
Some stems provide support by wrapping around a nearby plant or other support. Many
members of the Convolvulaceae are vines, and they have twining stems that wrap
especially well round wire fences.
Roots
Although most underground storage organs are modified stems, some tap roots, like
that of carrot (Daucus carota), are roots modified for storage. Another example is the
root tuber, which is a swollen adventitious root (sweet potato, Ipomea batatas;
cassava, Manihot esculenta).
Aerial roots occur on epiphytic orchids, and have a multiseriate epidermis called
velamen made of dead cells with suberized walls. It apparently functions both to
absorb water and nutrients during wet conditions and to retain water during dry
conditions. Some such roots are photosynthetic. The structure - function relationships
of these roots are complex and not well understood.
Pneumatophores (breathing roots) occur on some mangroves. They grow upwards
through the substrate, usually anaerobic mud, and bear lenticels that allow gas
exchange when exposed at low tide.
Prop roots are a type of adventitious root. In monocots they give the plant more
transport capability (monocots lack true secondary growth) as well as extra support.
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They usually arise from the lowest nodes and grow downwards through the air and into
the soil. Once in the soil they may contract a bit to help anchor the plant (corn, screw
pine (Pandanus).
Contractile roots shrink even more than prop roots, flattening and shrinking in bands
caused by cortex cells that change shape. These roots help to anchor the plant and pull
growing corms, bulbs, and rhizomes down, keeping them buried in the stable soil
environment away from herbivores. Examples of plants with contractile roots are
ginger, Canna, and perhaps surprisingly, coconut.
Root nodules, found mostly but not exclusively on legumes, are homes for Rhizobium
bacteria. These infect the host root via an infection thread, and are then encapsulated
in a nodule produced by the host. They receive nutrients from the host and in return
fix N2 gas to NO3G for the host to use. Species specific associations exist between host
and bacterium.
Haustorial roots are not true roots. Their anatomy is very different. They occur on
parasitic plants and invade the cortex and vascular tissue of the host plant. Some don't
touch the phloem, and belong to parasites that carry on photosynthesis (birdvine,
Pthyrusa stelis). Others invade the phloem; these parasites do not photosynthesize
much and depend essentially completely on the host for food (dodder, Cuscuta).
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8.
Grasses
Grass leaves consist of a strap-like blade and a basal sheath that enfolds the culm
(stem) of the plant. The sheath may be considered a flattened petiole; it is usually a
hollow cylinder split down one side. The sheath, like the blade, is photosynthetic. The
leaves of some grasses have a ligule, a white or brownish membrane that forms a
projecting flap or collar, at the juncture between the blade and the sheath. The base of
the blade may have auricles attached to it; these are claw- like projections that may
wrap around the culm. The basal blade tissue may contain wedge-shaped, collenchymarich (and thus flexible), hinge areas called dewlaps. Many grass leaf blades lack a
midrib (the corn leaf is an exception) and there is usually no stalk-like petiole
characteristic of dicot leaves. In many grasses, especially lawn or "turf" types, the stem
remains very short, with leaves growing from it. In other grasses, the internodes
elongate dramatically (corn, cane, and most notably, bamboo).
As in many plants, axillary buds form in the axils of grass leaves. These may elongate
to form axillary shoots. On short stemmed grasses, this proliferation of new shoots is
tillering. The production of new shoots leads to new leaf formation, and in the axils of
these leaves more axillary buds form. These in turn may elongate to form more tillers,
and so on. In this way, grass plants can spread very effectively. Young leaves are
normally tightly rolled around each other, forming a pseudostem. When the shoot
bearing an inflorescence elongates, it grows up through this pseudostem. The
inflorescence is enclosed by a large rolled-up leaf called a boot in crop plants. The
emergence of the boot covered inflorescence from the pseudostem is called booting.
When expanded, the boot becomes the flag leaf. It is the uppermost and best
illuminated leaf on the plant, and it makes the biggest photosynthetic contribution of
any leaf to the grass plant.
Morphology and Growth of a Grass Plant: Sugar Cane
(adapted from a class handout by J. W. Purseglove et al., 1962)
The cane may be considered a series of joints, each consisting
of a single node and internode. There is a band of root
primordia above each node, and above this band is a narrow
meristematic zone with an axillary bud at each node. When such
a joint (single sett) is planted, growth takes place thus:
1) Darkness and moisture influence roots to grow from the root
primordia.
2) The axillary bud grows, producing a short rhizome made of a
series of internodes, each succeeding one a little longer and
thicker than the preceding.
3) Adventitious roots grow from the root primordia on each node
of the developing rhizome, producing the root system.
4) When fully developed, the rhizome turns up and grows above
ground, producing leaves from a stem apex.
5) The stem elongates rapidly, producing a number of nodes that
are more or less constant for a given variety. A single leaf is
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borne at each node (alternate phyllotaxy), with a bud in its
axil.
6) Meanwhile, axillary buds on the rhizome produce secondary
shoots/tillers, tertiary shoots, and so on, these together
forming a stool.
7) The terminal bud on each shoot is finally transformed into
an inflorescence (a terminal panicle) and then that cane dies.
Thus, each cane may be regarded as a single monocarpic plant
perpetuating itself by means of rhizomes.
The rhizome/stool, bearing many plants of different ages, may
live for many years, producing new canes by the process of
ratooning. The cane ripens when fructose and glucose, located
mainly in the upper, growing part of the stem, turns into
sucrose. This accumulates towards the base of the stem, which
becomes economically the most valuable part of the plant.
Growth of Cereals
To put plant anatomy into the perspective of agricultural or crop botany, a good place
to look is the growth of cereal grasses. Successful management of cereals requires a
proper understanding of how they develop, and this in turn requires study of their
anatomy and morphology. Numerical growth scales based on progressive changes in
morphology have been developed by Zadoks, Feekes, Haun and others, to allow both
statistical analysis of cereal development (because they put development on an ordinal
scale) and unambiguous description of growth.
In the Zadoks two digit decimal code, ten principal stages in plant development (rather
than growth as an increase in mass) are identified, with each stage having as many as
ten secondary degrees of advancement. The scale recognizes that different types of
development can occur together by simply identifying the types of development as they
appear. Overlap occurs between seedling growth (1) and tillering (2), while for the
later stages, from stem elongation (3) to ripening (9), development is sequential.
Principal and Secondary Developmental Stages of the Zadoks Decimal Code
The principal stages are given in bold type and the identifiable secondary stages are shown
in parentheses.
Section
Developmental Stage
0
Germination (dry seed ÷ water absorption ÷ root emergence ÷ shoot emergence ÷
first leaf just visible)
1
Seedling growth (number of unfolded leaves on main shoot)
2
Tillering (number of tillers)
3
Stem elongation (number of detectable nodes, appearance of flag leaf)
4
Booting (extension of flag leaf sheath ÷ boot swelling ÷ flag leaf sheath opening)
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5
Head emergence (1st spikelet just visible ÷ ¼ ÷ ½ ÷ ¾ ÷ all of head emerged)
6
Flowering (beginning ÷ half way ÷ complete)
7
Milk development (kernel water ripe ÷ early ÷ medium ÷ late milk)
8
Dough development (early ÷ soft ÷ hard dough)
9
Ripening (kernel hard & indivisible ÷ undented by thumbnail. Overripe / straw dead
& collapsing. Seed dormant ÷ 50% germinating ÷ not dormant ÷ ready to plant)
Identification of secondary developmental degrees can be important. For example, the
right moment must be determined when to sow presoaked rice seed in direct seeding
by aircraft in order to maximize germination. Meiosis in grasses practically coincides
with stage 39, when the flag leaf collar (ligule) is just visible. In rice, this is when the
auricles of the flag leaf are at the same height as the next leaf, an important signal for
the rice breeder.
In the case of weed control with 2,4-D and certain other herbicides, it is essential to
time their application in relation to shoot apex development. Otherwise, abnormal
plant development or reduction in yield may occur. Minimal effect on plant
development occurs if application is made at, or soon after, the double ridge stage of
apical development. At this point both leaf and spikelet primordia are visible as a
double structure at each node on the shoot apex (just before the terminal spikelet
appears on the apex, stage 50). If applied earlier, during vegetative growth, leaf
abnormalities may occur. If applied later, floral abnormalities may occur.
In another example, for maximum dry matter growth of winter wheat in the face of
nutrient leaching by heavy rains, spring applications of nitrogen top dressing must be
made during early stem elongation (stages 30-31) to supplement soil reserves readily
depleted by the rapidly growing crop.
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9. Trees and Wood
A cross section of tree trunk will usually show a distinct outer ring of lighter sapwood,
containing functional secondary xylem, which surrounds a large disk of darker, non
functional secondary xylem called heartwood. The dark colour results from accumulated
tannins and phenolics, which can create a central core of still darker wood in some
trees. A thin band of secondary phloem, produced by an imperceptible layer of vascular
cambium, circles the sapwood, and this in turn is surrounded by a narrow layer of
cortex, and then the layers of tissue comprising the periderm layers of the outer bark.
The material normally referred to as bark is merely dead cork cells (phellem) on the
very outside of the tree trunk. In most trees this appears in characteristic patterns of
cracks, furrows, and blocks (see Chapter 3, Protective Tissue Layers for further
description of bark structure).
Generally "hardwood trees" (angiosperms such as mora, teak, and mahogany) are rich
in fibers, making them denser, heavier, and stronger than "softwood trees"
(gymnosperms such as Caribbean pine, Pinus caribbeansis; and Peruvian pine, Araucaria
sp.), which are poor in fibers and rich in tracheids. Structural strength of wood is
different from its durability. Hardwoods are generally more durable (long lasting) than
softwoods not because of their fibre content, but because of the chemicals, collectively
called extractives, with which they can be impregnated. These include flavinoids,
lignans, terpenes, phenols, alkaloids, sterols, tannins, sugars, gums, resin acids, and
carotenoids.
Growth rings
Concentric rings appear in the wood of a tree stump if there is annual variation in
growth caused by seasonal differences in climate, which cause the vascular cambium
to make secondary xylem at different rates. They are most obvious in trees growing in
temperate parts of the world, with distinct winters and summers. But growth rings
become progressively less obvious towards the equator, and even in Trinidad, with a
yearly wet season and a dry season, it is often difficult to see any difference in growth
of the wood in the tree. Growth rings, where present, are even apparent in the
secondary phloem and in the periderm. While growth rings, with some exceptions,
provide an accurate way to age trees, trees without growth rings are essentially
impossible to age.
Reaction wood
Trees produce reaction wood if they are leaning or have heavy branches. In angiosperm
trees this appears as wider growth rings on the side opposite the lean. This tension
wood is rich in gelatinous fibers that have abundant cellulose rather than lignin. These
fibers can contract to pull up and counteract the lean. Conifers have wide growth rings
on the leaning side that contains lignin-rich compression wood, which pushes up
rather than contracts.
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The presence of reaction wood decreases the value of forest trees as timber because
boards cut from them warp more during curing than do those cut from normal wood.
Some tree species growing on a hillside with merely a 10% slope may have reaction
wood in their trunks. This factor may help prevent or slow deforestation of the
Northern Range.
Rays
Rays provide a conduit across the radius of the tree trunk, from vascular cambium to
the center of the tree. The precise function of rays is not well understood, and given
their position, locked in a great mass of wood, they are inherently difficult to study.
However, the phloem rays, which traverse the tissue from the vascular cambium to the
cork cambium, apparently offer a living conduit for movement of substances and a site
for the storage of starch. Similarly, xylem rays, which traverse the wood from the
vascular cambium to the pith (if present), offer another pathway for lateral transport.
Some phloem rays appear to connect with xylem rays, while in other cases they are
distinctly separate.
Ring porous and diffuse porous wood
When the vascular cambium in a tree is producing secondary xylem a variety of cells
can form, including vessels, tracheids, fibers and parenchyma. In temperate regions,
large cells are generally produced in greatest abundance at the beginning of the
growing season when there is lots of rain, giving rise to rapid growth and hence large
celled early wood or spring wood. During the summer, with less rain, growth slows
and the cells produced are smaller, and the resulting wood is termed late wood or
summer wood.
An interesting extension of this simplified explanation for the banded appearance of
wood comes from consideration of certain trees with characteristic "porosities," another
way of saying vessel distributions. In diffuse porous wood (eg maple, apple, and
cherry), as the name suggests, large vessels are produced more or less uniformly
throughout the growing season, whereas in ring porous wood (eg oak, elm, chestnut)
large vessels are produced only early in the growing season, with smaller ones later.
These different patterns arise in these trees regardless of the weather, inferring that the
trees are opportunists, that is, vessels are not produced in response to abundant water
and nutrients but in anticipation of them. Consequently, ring porous trees may have an
advantage in areas with a short growing season in that they can capitalize on early
heavy rains, and diffuse porous trees may be able to deal with variable water
availability throughout the growing season.
Tyloses
A peculiar but fundamental feature of vessels in the secondary xylem of the wood of
some trees, are balloon like cells called tyloses. These are outgrowths of xylem
parenchyma cells into the vessels, through pits in the wall between the cells. Tyloses
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are living and can fill the vessels, eventually blocking transport permanently. They may
form as a result of injury, disease, or cavitation, and are of interest because they alter
flow in the xylem, thus affecting translocation and transpiration, and ultimately growth.
Also, they make it more difficult for wood to be infiltrated with preservatives in the
lumber industry.
Cambial activity in a tree trunk
The activity of vascular cambium in a tree trunk is nothing short of really really
complex. Consider this:
The vascular cambium in a tree trunk is a continuous cone of cells, narrow at the top
of a tree where the stem is young and small and secondary growth is just starting, and
then becoming bigger and bigger in circumference towards the base of the mature
trunk, perhaps a few meters around in a large tree. When a cambial cell divides, it must
produce a xylem cell or a phloem cell, and a cambial cell, or the process will stop and
the cambial cone will begin to disintegrate. The layer of cambium remains in position
towards the outside of the tree…producing a lot more xylem than phloem.
As the circumference of the cone of cambial cells gets larger, the cambial cells must
produce extra cambial cells between them, within the ring to their sides or, again, the
cone will begin to disintegrate. The number of xylem cells produced must exceed the
number of phloem cells in a precise ratio, or the ring of cambial cells will be pushed
inwards too far. Also, the production of xylem and phloem must be carried out by all the
cambial cells in the ring equally, but vary in rate from the top of the cone downwards
in a coordinated fashion.
And then, (if you are still with me) distributed in this ring are cambial cells called ray
initials that produce rays. Some produce phloem rays, some produce xylem rays, and
some produce both. At once. And you think your timetable is confusing.
The job of the cork cambium, in comparison, is much simpler….or is it? The cork
cambial cells also form a cone around the tree trunk, they also produce cells to the inside
that are different from those it produces outside, and since the cork (phellem) cells die
the cells beneath must guard against exposure to the environment, so the cork cells must
die and fragment in a controlled fashion. And the cork cambium does something the
vascular cambium does not: it regenerates in new layers underneath previous layers,
cutting them off and killing them, over and over.
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10
Flowers and their Modifications
Flowers and the fruit they produce are the most conspicuous of plant parts. They are
conspicuous, and attractive, because they are advertising the presence of nectar or edible
fruit to be exchanged for pollination or seed dispersal. At a fundamental level, what is
going on (at least to a geneticist) is that the plant's genes, wrapped in cells and
protected by embryos, seeds, and fruits, are using the rather elaborate vehicle of the
plant body to proliferate themselves.
The flower is the least variable part of the plant. It is altered less than leaves, stems, and
roots by environmental factors. This ensures that the plant's reproductive system
functions under a wide range of conditions, and that its genes are passed on intact. If a
flower is the wrong size or shape, it might be impossible for an insect to pollinate it.
A flower is a shoot system consisting of an axis and laterally borne leaves for sexual
reproduction. It differs from a vegetative shoot system because there are no buds in the
leaf axils, the internodes between the leaves are very short, and its growth is
determinate.
A flower consists of concentric rings of four kinds of leaves. Moving from the outside
inward, they are: sepals, petals, stamens, and carpels. Green sepals closely resemble
leaves, while petals and coloured (petaloid) sepals have a poorly developed vascular
system, no palisade mesophyll, little sclerenchyma, and chromoplasts instead of
chloroplasts. Most stamens and carpels do not look like leaves, but in the more primitive
flowers (Degeneria, Drimys, Magnolia, Michelia), they may be wide and flattened. It is
likely that stamens and carpels evolved from true leaves.
Petals and sepals are not necessary for reproduction; thus they are termed accessory
parts, while stamens and carpels are called essential parts. All the sepals together form
the calyx of the flower, and all the petals together make up the corolla, with both
whorls together comprising the perianth. The stamens (filament + anther) are the male
parts (androecium), and the carpels (ovary + style + stigma) are the female parts
(gynoecium).
Flowers arise in the axils of leaves. These leaves, which are usually small, are called
bracts. Some flowers have conspicuously coloured bracts that supplement the petals or
even replace them. Poinsettia, Bougainvillea, and Chaconia have brightly coloured bracts
and inconspicuous corollas. A single flower is borne on a stem called a pedicel, and the
point of attachment of the floral leaves to the pedicel is called the receptacle. The stem
of an inflorescence, a branched system of flowers, is called a peduncle.
Flowers of different species show great structural variety. These variations include:
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1. Loss of parts
A complete flower has all four whorls present; an incomplete flower lacks one or more
of them. A perfect or hermaphrodite flower has both carpels and stamens, but may lack
sepals, or petals, or both. An imperfect flower lacks either carpels or stamens. A
staminate (male) flower has only stamens, no carpels; a pistillate (female) flower has
only carpels.
On a corn plant, the ear is a carpellate imperfect flower. Its silks are greatly elongated
styles, with sticky stigmatic surfaces for catching airborne pollen. The tassel is a
staminate imperfect flower. Corn is monoecious ("one house") because it has both male
and female flowers on the same plant. Coconut and most cucurbits are monoecious, too.
Papaya (Carica papaya) is dioecious ("two houses"), with male and female flowers
borne on different plants. Mango inflorescences (Magnifera indica) have both male
flowers and hermaphrodite flowers, and the presence of numerous male flowers helps
explain why the tree produces far fewer fruit than flowers.
2. Fusion of parts
Floral parts that appear fused arise together that way, rather than arising separately and
then fusing. Perhaps they were separate in primitive ancestors. Within whorls, if the
sepals are fused to form a tube, the flower is gamosepalous (Hibiscus). If the petals are
fused, it is gamopetalous (Allamanda). If the petals are free (no fusion) it is
polypetalous. The usual fusion between whorls is between stamens and sepals; such
stamens are adnate to the petals, producing an epipetalous flower.
If the filaments of the stamens are fused into a tube, they are connate, and the
androecium is aldelphous. If the carpels are fused, the gynoecium is syncarpous, and
if they are free, it is apocarpous. Fusion of gynoecium and androecium produces a
gynostegium. in , the petals are fused. Many legumes (of the Papilionoideae) and all
Compositae have their stamens fused to form a tube.
3. Ovary structure
Ovules arise from swellings on the inside wall of the ovary. These swellings are
placentas. A simple ovary, which contains only one carpel, always has marginal
placentation, with the ovules arising along the junction of the two margins of the
carpel. If there is more than one carpel, this kind of placentation becomes parietal. If
the ovules are attached to the central axis formed by the carpels, it is axile. In central
placentation, there is only one locule, and the ovules are borne on a central axis, and
in free central placentation the axis is incomplete. In basal placentation there is also
one locule, and the ovules are attached to the base of the ovary.
4. Ovary position
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Flowers that are hypogynous, meaning that the other flower parts are “below the
gynoecium," have convex receptacles. The ovary is superior to the rest of the flower. A
perigynous ("around the gynoecium") flower has a concave receptacle, so the ovary is
below the other floral parts, and is half superior. In epigynous ("above the gynoecium")
flowers the ovary is inferior and embedded in the receptacle. Here, the tissues of the
receptacle and ovary are fused and indistinguishable from their primordial beginnings,
so that the mature ovary wall of the fruit (pericarp) and the receptacle tissue together
create a false or accessory fruit.
5. Aestivation
Flowers show variation in the horizontal arrangement of their calyx and corolla
(aestivation). In a valvate flower, the petals or sepals meet without overlapping. In a
contorted or regular flower, the petals or sepals all overlap in the same direction. In an
imbricate or irregular flower, the petals or sepals overlap in both directions, so that one
sepal or petal is wholly inside the ring, and at least one other is wholly outside the ring.
6. Floral shape
Actinomorphic flowers exhibit radial (multilateral) symmetry. All the floral parts in
each whorl are alike in size and shape, and any longitudinal cut across the center creates
two mirror image halves (Hibiscus, Cucurbita). Zygomorphic flowers have bilateral
symmetry, and only one specific longitudinal cut across them will produce mirror image
halves. This symmetry results from differences in the size and shape, or loss, of some of
the parts (Orchidaceae, Leguminoceae). In asymmetric flowers there is no symmetry
to the arrangement of the parts (Canna).
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11
Agents of Pollination
Flowers are pollinated in many ways, and it is often quite easy to tell what pollinates a
given flower by its appearance. Tiny, reduced flowers are often pollinated by the wind.
The plant has no control over where the wind carries the pollen, and most of it will
probably fail to reach another flower. The wind is not attracted by beautiful blooms, so
it is wasteful to produce them. The plant devotes its metabolites to producing great
quantities of very light pollen, not showy flowers. The grasses as a group are wind
pollinated, as are many temperate trees.
To ensure a greater success rate in pollination, many flowers enlist the help of insects,
birds, or bats. Flowers and pollinators have coevolved for millions of years, sometimes
developing species-specific associations, reflected in the great variety of floral
morphology. Many orchids and legumes have zygomorphic flowers (bilaterally
symmetric) because they physically interact with the insects that pollinate them.
Of the insects, bees and wasps, with more than 20,000 species, form the largest group
of pollinators. The colours these insects perceive differ from those we see, and the
colours of wild flowers are for the insects' benefit, not ours. Honey bees can see
ultraviolet light that is invisible to us, and red looks black to them. In general, bees are
attracted to sweet, fragrant flowers that look yellow or blue to us.
Beetles are attracted to flowers with strong, yeasty, spicy or fruity smelling, white or dull
coloured flowers. These flowers may have petals or other parts, that are edible, rather
than nectar. The flowers of the cannonball tree (Couroupita guianensis)have sets of
fleshy, edible decoy stamens, with little or no pollen, to attract beetles. While feasting
on the decoy stamens, pollen is brushed on the beetles' backs from real stamens
arranged above them.
Red brown, foul-smelling flowers attract flies. The carpels and stamens of dutchman’s
pipe (Aristolochia grandiflora) are enclosed in the base of a large trumpet-shaped floral
tube. Flies are attracted to the trumpet by its red brown colour and by the carrion odour
produced in the early morning. Flies entering the mouth of the trumpet are led inside
by inward pointing trichomes until they encounter a septum perforated by a small hole.
Light from the thin-walled floral chamber on the other side entices them to go through
the hole. As they seek an exit, they crawl over the stamens and carpels. The flies remain
trapped for about two days until the stamens, which ripen after the carpels, mature and
shed pollen onto the flies. Then trumpet turns downward, allowing the flies to escape
so they can haplessly seek other blooms to cross pollinate.
Moths are attracted to fragrant, white or yellow flowers that are visible at night. Flowers
pollinated by butterflies tend to be large and red, with tubular corollas having nectaries
at their base. Only butterflies have tongues long enough to reach the nectar.
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Flowers pollinated by hummingbirds are often red, odorless, and have floral tubes that
are even longer than those of butterfly-pollinated flowers. These exclude insects and
allow only hummingbirds to feed on the nectar, which is produced in large quantities
because the birds survive exclusively on it. These birds cannot smell, and the flowers,
which are usually red or yellow, tend to lack scent.
Bat-pollinated flowers tend to be large, dull coloured, and open only at night. Because
bats may have trouble finding flowers amongst foliage at night, they are often borne on
branches away from leaves. The flowers of the sausage tree (Kigelea africana) exhibit all
of these features beautifully.
Some members of the orchid family, which comprises more than 35,000 species, show
extreme modifications to attract pollinators. A number of species are completely
dependent on certain species of bee, on a one to one basis, for pollination. Many orchids
have sticky pollonia (pollen sacs) that stick to bees' heads when they visit the flower.
Both bee and flower must be the right size and shape for the transfer to work, and if the
right bee species does not visit the flower, it does not get pollinated. The flowers of
some of Orphys sp. look like female bumblebees. Male bumblebees try to mate with the
flowers, but they pollinate them instead.
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12
Fruit and Seed Modifications and Dispersal
Of the fleshy fruits, the berry (e.g., tomato), pepo (e.g., cucumber), and hesperidium
(all citrus) have a pericarp that is partly or entirely edible, attracting the attention of
frugivores that aid in the dispersal of their seeds. These are protected against teeth and
digestive enzymes by woody testas made of sclereids.
The drupe is different. The innermost layer of the pericarp, the endocarp, is made of
fibers or sclereids, so the fruit wall contributes to seed protection. Some drupes, like the
almond, are dispersed by bats. Bats often pick almonds from the tree on which they
grow and take them to a favorite roost tree, where they consume the edible exocarp and
mesocarp, and discard the fibrous endocarp, the seed safe inside, thus effecting seed
dispersal.
The coconut is a drupe so large that frugivory is meaningless as a vector for dispersal.
Coconuts do get dispersed by the vagaries of ocean currents, however, because the air
filled, fibrous mesocarp and the "shell" (endocarp) made of sclereids keep salt water
away from the seed, allowing a voyage of perhaps some hundres of kilometers.
The fact that a coconut is classed as a drupe and is therefore a fleshy fruit may make
more sense if you look at the fruits of the oil palm and of the ornamental dwarf palms.
These are somewhat "fleshy" and more obviously drupes, and this classification extends
to include the coconut. Botanically, the parts of a coconut are (moving inwards): green
covering, exocarp; fibrous, air-filled husk (coir), mesocarp; shell, endocarp; brown layer
inside endocarp, testa; "meat" (copra), seed. Coconut water is liquid endosperm.
The pericarps of dry fruits either split open (dehisce) to release the seeds within, or
remain closed at maturity and are termed indehiscent. Indehiscent fruits remain closed
because the carpels are fused too tightly together to split, but the "sutures" joining the
carpels of dehiscent fruits separate more easily. Follicles have one carpel; this splits
down one side at maturity leaving the seeds in a cup like structure. In the milkweed
follicle the seed testas have long hairlike trichomes that become plumes in the wind to
float the seeds away.
Legumes have one carpel that separates at maturity along two sutures, sometimes
explosively, sending the seeds far from the parent plant. Siliques have one carpel that
separates along two sutures from a central partition to which the seeds remain attached
until they are mature. They are freed by wind blowing the plant, encouraging dispersal.
Capsules have more than one carpel; these split apart in a variety of ways to release the
seeds. The capsule of the poppy resembles a censer, and the seeds are scattered when
this is blown about by the wind. The mahogany fruit is a type of capsule. When it splits
open it remains attached to the tree and its seeds, which have winged testas, rotate and
float away from the tree.
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In many dehiscent fruits, dehiscence results from layers of fibers in the pericarp
shrinking in opposite directions during maturation, until the carpels are broken apart.
The testas of the seeds within are often made of sclereids. Legume testas consist of only
two layers of sclereids.
Although indehiscent fruits do not split apart to release their seeds, they have a number
of modifications for dispersal. The samara (e.g., Myroxylon) has a winged pericarp so
that these fruits can fly away like the winged seeds of mahogany. In the caryopsis (or
grain) the pericarp is completely fused to the testa. Many grains are small and light and
enclosed in floral bracts that trap air, so they are readily dispersed by the wind. Such
grains are also dispersed by water. Achenes are like grains except that the pericarp and
testa do not fuse. They too are often small and light and easily wind-dispersed. The
pericarps of nuts are thick and woody (primarily sclereids), making them too heavy for
wind dispersal. But nuts do attract the attention of rodents, which hoard them, at the
same time often forgetting where, and many trees get planted this way.
Pericarps and testas may have trichomes or emergences modified into hooks or burrs or,
alternatively, they may have sticky surfaces, all designed to engage animal fur or hikers'
socks as the vector of dispersal. The water lily has a spongy outgrowth of the seed (the
aril) for flotation. Some arils are edible, such as that on the seed of akee, attracting
frugivores, while the seed itself is indigestible. The akee is an example of a fleshy
capsule.
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13
Making Biological Drawings
Biology students frequently have difficulty with effective use of the microscope and with
making effective drawings of specimens, typically of material they have had to section
by hand for observation. Here are some tips and guidelines to help make these activities
more rewarding.
It is easy when faced with a specimen, an intact plant or a section of it under the
microscope, to get carried away and attempt to record too much of it in a drawing. The
key is to start with a faintly executed sketch showing the outline of the specimen in
correct perspective and proportion, then add necessary details without going overboard,
and finally draw over your original lines with a bolder line, erasing the original sketch
as you go.
Start with a sharp pencil and keep it sharp. If it becomes dull, the line will become broad
and faint and you will have to press harder to make a mark, and risk driving the point
into the paper, which making erasures difficult. Sharp pencils, like sharp knives, are
easier to control. The line goes where it should. With regard to hardness, B or HB grade
ought to be best, but grades vary among brands, so find a good brand and stick with it.
You want to make solid black lines that don’t easily smudge.
Use good quality paper. It must be thick enough to prevent drawings on consecutive
pages being visible through the paper. To keep your drawing clean & smudge free, draw
as much as possible from the top down. Avoid resting your hand on the drawing. When
you have to add details to a drawing, use a sheet of paper as a shield. Have a good
“plastic” eraser (e.g., Staedtler®) handy. When erasing, use the paper shield again. It
helps too in isolating the area you are erasing. If the eraser is dirty, it will transfer the
dirt to your drawing, so rub it on a clean scrap of paper first.
Always aim for realism in your drawing rather than accuracy. If the specimen you are
drawing is a section of fresh material, rather than a prepared slide, your section will be
unique and it will do no one much good to have all the details of your specimen in the
drawing. In this regard, never attempt to draw trichomes or root hairs. They will not add
to your drawing and usually make it look like some kind of insect.
If you are making a tissue map, do not draw cells. It is important to draw what you see,
while correcting any distracting defects your specimen. Breaks or tears in the tissue
reflect sectioning problems and these must not be represented in your drawing. A good
botanical drawing does not have to be photographically perfect.
Instead, it is a diagram showing the outlines of the important features. It smooths out
and eliminates distracting imperfections, while retaining the shape, proportion, and
distinguishing characteristics of each feature. Indeed, a good botanical drawing can be
an improvement on a photograph. Drawing allows you to emphasize areas of interest,
leave out extraneous details and flaws, and integrate a mass of structures into a realistic
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impression of the actual material, rather than simply but uncritically photographing the
lot.
In a tissue map you are not required to draw cells, so concentrate on outlining each area
of tissue. If your specimen is a cross section of a dicot stem in primary growth, you will
see a lot of vascular bundles and they will all be different. Although usually the same
tissues occur within each of the vascular bundles, each bundle will appear unique
because the amount of each type of tissue is different in each bundle. Therefore, draw
them to show the differences they exhibit.
Show all the tissues in your section in correct proportion to each other. It is a big
temptation to draw a series of concentric circles like an archery target and think that
constitutes a tissue map, but if you take the time to properly represent each area of
tissue, the result will be much better.
It should take less than 10 minutes to draw a good tissue map. If you find that after 30
minutes you are still not finished, you are probably worrying over small details instead
of just trying to get a general sense of the arrangement and appearance of the tissues
and drawing that. Remember, because every section is different, you do not have to be
concerned if this or that vascular bundle is not accurately depicted, or if the shape of the
stem section is not just right.
Your drawings must have a title at the top of the page (e.g., "Cross Section of Coleus
sp. Stem in Primary Growth"). Labels must be neatly printed, not written longhand.
PRINTING IN BLOCK CAPITALS IS LIKE SHOUTING. It is ugly and hard to read, so just
capitalize initial letters of words where it is appropriate to do so. As far as possible,
labels should be grouped on the right-hand side of the drawing.
Scientific names, being Latin, must be underlined to indicate they are in a foreign
language. Capitalize only the first letter of the genus, and nothing in the species name.
It is a convention to abbreviate the initial letter of the genus in the binomial when it is
used more than once. Include the approximate magnification of your drawing (refer to
"Instructions for Using the Microscope" to calculate this).
Finally, presentation counts. If you are compiling drawings in a book, take every effort
to keep it clean and all entries neat. Any extraneous notes or graffiti will detract from
it. Make a Table of Contents and number the pages.
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14
Making Hand Sections of Plant Material
With a little practice it is easy to make sections of fresh plant tissue, and there are
advantages to studying sections of living material rather than of prepared material.
When making sections of plant material, you must use a new razor blade. It is essential
to have a straight from the package, a never-before-used, double-edged razor blade. The
one you used last week, or the one you used a few minutes ago will only do now for
trimming cuts. Single edged, Gem® type blades are too rough for thin sections, and so
are scalpels. Keep the blade wet, and keep the specimen you are cutting wet. Do not
laboriously make one section, stain it, and then look at it under the microscope, just to
find out that it is no good. Instead, first wet the blade and use a rapid sliding or swiping
motion (rather than chopping) to cut many (5-10) sections in rapid succession all at
once.
The resistance met by the blade in the tissue will tell you how to make your cuts, and
you will find that the very first few sections made with a new blade are better than the
rest. Use part of the blade for rough cuts, and save the rest for thin sectioning. If the
material is thick or tough, try sectioning part of it and then draw a representative
complete section from it. Immediately transfer your sections all together from the side
of the blade to some water in a watch glass or staining block.
Sections which look opaque are probably too thick. Sections which are not cut at a right
angle to the plant surface will be no good. Select a few of your best sections and mount
them in a drop, not a puddle, of glycerine on a slide, and cover with a cover glass,
lowering it at an angle to let air bubbles escape. Do not mount sections in water. If the
sections dry out they will be ruined, so work quickly.
If sections are to be stained, typically to show lignin, this must be done in a watch glass,
never on the microscope slide. Newly made sections should be transferred straight from
the side of the razor blade to the stain. When using phloroglucinol HCl, lignin, found in
the tracheids, vessels, fibers and sclereids in your sections will turn pink or red. Usually
your sections must be thin to prevent over staining, but if the lignin deposits are sparse
then thicker sections will stain better. Leave them in the stain about a minute, then
mount in glycerin on a microscope slide. Never mount sections in stain, or you will have
a real mess.
While many stem and root specimens are rigid enough to hold and section freehand,
leaves are usually more difficult to deal with. It is essential to use a never before used
razor blade and to keep it wet when sectioning. One of these methods should prove
effective for you:
Method 1: The conventional way to section a leaf is to cut part way through a small
block of carrot root and insert the piece of leaf you want to section in it. Then, section
through both carrot and leaf at once. Alternatively, use pieces of Styrofoam® to hold the
leaf.
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Method 2: Make a series of closely spaced parallel cuts in the area of leaf you want to
examine. Then free the sections all together from the rest of the leaf with two cuts made
across the parallel ones.
Method 3: Holding two blades together, cut a few pieces from a part of a small leaf
containing the area of interest. Your sections will lie between the two blades.
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15
Instructions for Using the Microscope
Designs vary, but in general a microscope consists of a stand comprising the foot,
specimen stage, an arm, a body or tube containing the image forming optical system, a
substage condenser, and a light source with an integral or separate, variable transformer.
The function of the condenser is to focus the parallel rays from the light source onto the
specimen mounted on a glass slide and moved about by a mechanical specimen stage. An
iris diaphragm in the condenser is used to adjust image contrast.
The tube is a cylinder with prisms in it and an eyepiece (ocular) lens system at the top.
At the base of the tube is a revolving nosepiece to which are attached objective lenses of
different magnifications. Objects are brought into focus by the vertical movement of the
tube or specimen stage controlled by the coarse adjustment (rack and pinion) and fine
adjustment. A microscope is a precision and delicate instrument. Treat it with care.
Operation
1. Carefully place the microscope on the bench and turn on the light source.
2. Open the condenser iris fully using the lever provided and then raise the condenser.
On some microscopes this can be moved with a knob, while on others the condenser is
twisted in a helical sleeve to adjust its height, or turned to click on different preset
aperture openings. Make sure you understand how it works.
3. Using the X10 objective obtain a uniform illumination of the field of view by adjusting
the iris diaphragm. Make sure you are familiar with its operation. The condenser is often
used, and wrongly so, to control the amount of light illuminating the specimen. Its real
purpose is to control image contrast and resolution.
4. Mount the piece of material to be examined on a slide with a small drop of glycerine
and, using a needle, cautiously lower a cover slip onto the specimen to exclude air
bubbles. Never mount specimens in stain.
5. Place the slide on the stage with the specimen directly below the X10 objective and
bring it into focus using the coarse adjustment. The focal length of the X10 objective
(i.e., the distance between lens and object when in focus) is about 0.5-1 cm.
6. Adjust the condenser iris diaphragm to give a satisfactory compromise between
resolution and contrast.
7. To view the object under the high-power X40 objective, place it in focus in the center
of the X10 field and swing the X40 objective into position. The distance between the
objective and the cover slip is very small, so bring the object into focus using the fine
adjustment only. On some microscopes the fine focus control has limited travel. Before
using this kind of fine adjustment rotate the knobs until they appear equally spaced on
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either side of the arm.
NEVER RAISE the stage or lower the tube when the X40 objective is in place while you
are looking at the image through the eyepiece. The condenser will still be in focus on the
specimen and will require little adjustment; the iris diaphragm will however, probably
need some readjustment.
Do not keep on turning the fine adjustment if nothing appears. Ask for help. Remember
to adjust the condenser settings and light level for the best image.
To obtain the best results from your material and prepared sections, it is essential that
you follow the above procedure each time you set up your microscope.
Points to remember
e Always remove surplus liquids from the surface of the microscope slides before
putting them on the stage.
e Avoid spilling liquids on the stage and always keep the lenses dry and clean. If the
image looks murky and you think the lenses might be dirty, ask a demonstrator for help.
e To form a three-dimensional picture of the specimen that you are examining, move
the fine focusing adjustment up and down fairly rapidly.
e When making drawings from the microscope, have your paper close to the microscope
and on the right side (if you are right-handed), and constantly refer back from your
drawings to the microscope.
e Try to keep both eyes open when looking down the microscope. This saves much
eyestrain. Do not stare fixedly at the image. Gaze in the distance often to rest your eyes.
e Microscopes must always be carried upright with one hand under the base. DO NOT
SWING the microscope like a basket.
e Handle cover glasses by the edges only. Take only a few (3), not dozens, from the
stock.
e Treat prepared slides with care. They are expensive and difficult to replace.
e Ensure that no slide remains on the microscope stage when you are finished.
Magnification of drawings
The magnification of your drawing has nothing to do with the total magnifying power
of the lenses used to observe the specimen you drew (eyepiece magnifying power times
objective magnifying power). It depends on the dimensions of the specimen and the size
of your drawing. For example, if you make a drawing of a cross section of a stem that
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is 3 mm wide, and your drawing of it is 150 mm across, the magnification of your
drawing is X50. If you draw a cell that is 200 :m long and your drawing of it is 10 cm
long, what is the magnification of your drawing (is X500 correct?)
If your microscope has a graticule scale in its eyepiece you can use it to measure things
if you calibrate it with a stage micrometer, which is a scale with divisions that are
usually 10 or 100 :m apart. The calibration procedure simply consists of counting how
many divisions on the stage micrometer correspond to a given number of graticule scale
divisions, and working out the micrometers per division of the graticule scale for each
objective on the microscope.
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Exercise 1
Anatomy of Herbaceous and Woody Stems
Coleus stem apex
Examine the Coleus plant. It has two leaves per node that are directly opposite to each
other at 180°. Pairs at successive nodes occur at 90° (opposite decussate phyllotaxy),
so that you see four rows of leaves if you look directly down on the plant. Because of
this, you will be able to see only half of the leaf primordia and developing buds in a
median longitudinal section of the apex.
Obtain a prepared slide of a longitudinal section of Coleus stem tip, and observe it using
the X4 objective of the compound microscope to obtain an overall view. The leaf
primordia (note that the singular form of primordia is primordium) occur in pairs and
you will probably have two pairs cut in longitudinal section on your slide. On the apex
used to make the slide, there was another pair of primordia located between the two
pairs included in the slide. All that can be seen of the intervening leaf primordia are two
bulges of parenchyma cells on the stem just below the youngest pair of leaf primordia.
View the apex at higher magnification. Note that most of the cells are parenchyma. The
meristematic cells are conspicuous because of the high nucleus/ cytoplasm ratio, and the
differential staining of the nucleus and cytoplasm.
Locate the apical meristem of the stem and the axillary bud meristems (at the bases
of the largest leaf primordia). Below the apical meristem, the parenchyma cells are still
dividing. This region is the primary, elongating meristem, or subapical meristem.
Look further down, in the middle of the stem, between the largest primordia, for a small
patch of flattened or squarish cells. These cells, which form an intercalary meristem,
divide to add length to the stem, and cause internodal regions to form between the
leaves in the mature shoot. Some mature vascular tissue may be seen in the larger leaf
primordia; procambium cells are found in the same location in the younger leaf
primordia.
Draw the apex, and make sure you show the locations of the various regions described
above. Remember that you can't get two identical longitudinal sections of the apex, so
you might not find everything described here. DO NOT DRAW CELLS, or trichomes
(some of which appear as disconnected circles or ovals).
Primary Growth
Stem anatomy of Coleus
Cut thin cross sections from an internode of Coleus near the apex. Refer to Making Hand
Sections of Plant Material for instructions on proper technique. Immediately transfer
these all together from the side of the blade to some phloroglucinol in a watch glass.
After a minute or so add to this a drop of concentrated HCl. Lignin, found in the xylem
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vessels and fibers in your sections, will turn pink or red. Your sections must be thin to
prevent over staining. Leave them in the stain about a minute, then mount the thinnest
one in a drop, not a puddle, of glycerine on a microscope slide. Remember: Do not
mount sections in water or stain, or you will have a real mess.
Make a realistic tissue map drawing of your best section. DO NOT DRAW CELLS. Make
sure you recognize, and draw: epidermis, cortex, collenchyma (if present), bundle cap
of fibers (if present), phloem, cambium, xylem, and pith. Because you are using fresh
material, everybody's section will be different.
Stem anatomy of Zea mays
Obtain a prepared slide of Zea mays stem in cross section, and draw a tissue map of it.
DO NOT DRAW CELLS. Make sure you recognize and draw: epidermis, exodermis, and
vascular bundles distributed in a matrix of conjunctive tissue. Like monocots in
general, it lacks true secondary growth. Therefore, many vascular bundles occur in your
cross section, instead of the ring of vascular tissue, common in dicots, which becomes
more densely organized as secondary growth proceeds. Although the vascular bundles
may seem randomly distributed, their arrangement is highly ordered and groups of
bundles are destined to lead into specific leaves on the plant.
Corn, like all grasses, has one leaf per node arranged at 180° at successive nodes
(alternate, or distichous phyllotaxy), forming two rows of leaves when seen from
above. All of the vascular bundles appear remarkably alike. Each consists of two large
metaxylem vessels, a protoxylem lacunar space (which may appear as parts of one
or two vessels), phloem, and a few associated cells, all wrapped in a lignified bundle
sheath. Draw one vascular bundle, showing the xylem and phloem.
Secondary Growth
Coleus
Compare the tissues in the older part of the younger stem you looked at first. The cortex
and pith are now separated by a band of lignified cells, composed of fibers and large
xylem vessels, with phloem outside the lignified cells and separated from them by a
layer of cambium. Look for phloem fiber cells, (also called a bundle cap of fibers),
collenchyma, and a prominent epidermis with a thick cuticle. Part of the cortex is
composed of chloroplast filled cells. Some of it is collenchymatous. Make sure you
include these areas in a tissue map drawing of your section. Do not draw cells.
Rhipsalis
Make a thin cross section of the photosynthetic stem of Rhipsalis and stain it with aniline
sulphate. Gently agitate the sections to accelerate staining. Mount your best sections in
a drop of glycerine (not water or stain) on a slide beneath a cover glass.
The vascular tissue is arranged in a band separating the cortex and pith, with cortical
bundles (leaf traces) scattered in the cortex. This plant does have leaves, but they are
very small. The circular spaces in the cortex are really not large cells, but ducts. Each
duct is surrounded by a ring of secretory cells. Look for a thick cuticle covering the
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© 2006 G. F. Barclay - Plants Alive
epidermal cells, and for underlying cells full of chloroplasts. Identify the xylem vessels,
phloem, and thick-walled fibers. The xylem and phloem are separated by a ring of
cambium that appears either unstained or slightly pink when treated with aniline
sulphate. The pith cells of your section might contain druse crystals or starch grains.
Draw a tissue map of your section. Do not draw cells.
Wood
Everyone is familiar with growth rings in trees, but it may not be so obvious what these
represent. Each "dark ring" is not really a distinct kind of tissue, but a region of
secondary xylem laid down by the vascular cambium during the dry season. Growth is
slow then and therefore the cells are smaller, and these appear as the dark ring. Rings
of light wood form during the wet season when growth is rapid and the cells become
larger.
Tree trunk cross sections
Examine the cross sections of the trunk of a pine tree to see the growth rings in its wood,
and the thick layer of bark, which also has growth rings. The bark is deeply fissured
because it grows more rapidly in some areas than in others. There are lenticels at the
base of the fissures where the bark remains thin. Some lenticels appear on the surface
of the thick bark. These formed when the tree was young and they are now
nonfunctional.
Sambucus canadensis young stem
Examine a prepared slide of this stem and make a tissue map to show: epidermis, with
periderm developing beneath it, collenchyma, cortex, phloem fibers, a ring of phloem,
another ring of flattened vascular cambium cells, then a third ring, this consists of xylem
vessels and fibers. Finally, an area of pith fills the center of the stem.
Tilia stem
Examine a prepared slide of Tilia wood (use either a "1, 2, 3 yr" OR a "stem
combination" slide). In these progressively older stems you will see concentric bands
of secondary xylem, each separated a growth ring consisting of smaller cells. In the
center of each stem is a region of pith produced by primary growth during the first year
of the tree's life. Look for a layer of vascular cambium outside the xylem. Outside the
cambium is the phloem, but this is poorly preserved in your section because drastic
procedures such as boiling in acid must be used to make the wood soft enough to
section, and the phloem cannot survive this.
However, phloem fibers do survive, and outside the fibers you will see periderm being
produced. Look for a layer of flattened cells comprising the phellogen (cork cambium),
producing phellem (cork) outward and phelloderm (which contributes to the thin band
of cortex) inward. Radiating outward from the edge of the pith, like spokes on a wheel,
you will see rays. They allow lateral transport across the stem, and some of them end
at the cortex in triangular patches of thin walled cells. These arise from an unusual
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kind of secondary growth. Make a tissue map of a 3-year-old stem showing the features
mentioned.
Tracheids
Examine the demo slide showing tracheids in Pinus wood. The xylem of conifer wood
has no vessels, just tracheids. They have thin lignified secondary walls and pointed ends.
Among the tracheids you will see radial rows ray cells. Do not draw.
Lenticels
Examine the demonstration slide of lenticels in Sambucus bark to familiarize yourself
with their structure. Do not draw.
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© 2006 G. F. Barclay - Plants Alive
Exercise 2
Leaves and Roots
Leaf Anatomy
Allamanda cathartica - a dicot
Make a detailed tissue map of the entire midrib (and some of the adjacent leaf blade).
Be sure to include: epidermal cell layer on the upper leaf surface with heavily cutinised
outer walls, a layer of chloroplast-filled palisade parenchyma cells with a gap directly
over the vascular tissue of the midrib, filled with collenchyma cells, a layer of phloem
tissue, files of xylem vessels, another layer of phloem tissue, isodiametric parenchyma
cells, another layer of collenchyma and finally, on the lower surface of the leaf, a layer
of lightly cutinised epidermal cells. Is the vascular tissue surrounded by a bundle
sheath? Pay attention to this in your drawing. The circular spaces ringed by chloroplastfilled parenchyma cells in the midrib are laticifers, which conduct a kind of latex. Do
not draw any cells.
Zea mays - a monocot
Examine a prepared slide of a Zea mays leaf in cross section and draw a tissue map of
the thick part of it, showing the distribution of the vascular bundles within the
mesophyll. Do not draw all the cells. Some areas of the epidermis, and the adjacent
mesophyll, are lignified. Where lignified mesophyll cells join the epidermis to vascular
bundles they are called bundle sheath extensions. Like many grass leaves, the
epidermis contains files of bulliform cells. You should find groups of them, which in
cross section appear as bulges composed of vertically elongated cells. They are thought
to lose turgor if the leaf is under water stress, causing it to roll up, presenting less
surface to the drying influence of sun and wind, thus conserving water.
While the larger vascular bundles are surrounded by a lignified bundle sheath (and
resemble stem vascular bundles), the smaller bundles are surrounded a double ring of
mesophyll cells. This is the Krantz anatomy (Krantz is German for wreath)
characteristic of C4 plants like corn. You should be able to see conspicuous chloroplasts
in the ring of large cells, which are important in C4 photosynthesis.
Leaf types
Study the demonstration slides showing three types of leaves. One type is the
mesophytic leaf, adapted to an environment that is neither too wet nor too dry. The
second type is the hydrophytic leaf, adapted to an aquatic environment. This is the
floating leaf of Nymphea, with large gas-filled aerenchyma tissue toward the side in
contact with the water, stomata on the side in contact with the air, striking star-shaped
astrosclereids, and a thin epidermis. The third type is a xerophytic leaf, adapted to a
dry environment. Note the very heavy cutin layers on both sides of the leaf, an elaborate
upper epidermal layer of large, thin-walled cells, and an isolated palisade parenchyma
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© 2006 G. F. Barclay - Plants Alive
layer. Do not draw.
Stalked glands
Study the demonstration slide of a whole mount of a stalked gland of Drosera (sundew
plant). This is one of the most striking leaf adaptations and it is designed to attract, kill,
and digest insects.
Venation patterns and stomata
Study the prepared slides showing square pieces of cleared leaves of Z. mays and
Hibiscus to see the differences in venation and stomatal structure between monocots and
dicots. Focus carefully down through the leaf and you will see the different tissues.
Stomatal crypts
Study a prepared slide of Nerium oleander leaf in cross section. The stomata are confined
to trichome-filled crypts, reducing the rate of transpiration during photosynthesis, and
allowing this plant to survive in a dry environment.
Phyllotaxy
The leaves on many plants are arranged in a spiral around the stem. The object of this
exercise is to measure the angle between the leaves on a number of plants. Work in
groups as directed and use the assigned plants. On each plant, use a protractor to
measure the angle that each of the leaves makes with regard to its adjacent neighbour.
Place the axis of the protractor over the center of the severed stem and line up the zero
degree line along the petiole and midrib of a leaf. Measure the angle of the petiole and
midrib of the next leaf. Compile a list of the leaf angles of several leaves on each plant
and find the average. The data from the entire class will be compiled and compared.
Roots
Ranunculus - a dicot
The slides of this root show both younger and older forms. Draw tissue maps of each
form. The younger root has an indistinctly developed epidermis, round cortical cells,
an indistinct endodermis, a few protoxylem vessels arranged in bundles with phloem
lying between them, and a few "pith" cells (really future metaxylem vessels).
The older root has a distinct epidermis, cortical cells filled with starch, a lignified
endodermis, a pericycle about one cell layer thick and tetrarch xylem with phloem
between the arms of vessels. Each of the tetrarch xylem arms has protoxylem vessels at
the tip, and metaxylem vessels toward and within the central hub of metaxylem. There
are unlignified passage cells in the regions of endodermis opposite the protoxylem
arms. Your root section might include a lateral root or a lateral root primordium. Where
does it arise?
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Corn
Obtain a prepared slide of Zea mays root. Make a tissue map of your section, and
identify the following: epidermis, exodermis inside the epidermis, cortex, endodermis
(innermost layer of cortex, with lignified walls in this case), pericycle, xylem vessels,
phloem, and pith. The exodermal cells, which comprise the outermost layer of the
cortex, have a square profile in this prepared slide, but not much obvious secondary wall
development. The phloem may be difficult to find. Ask a demonstrator for help. Do not
draw all the cells.
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Exercise 3
Crop Plant Anatomy and Morphology
Cane and Corn
Many grasses are very small, making them difficult to study, but fortunately grass
anatomy/morphology is remarkably similar regardless of the size of the species.
Therefore, the large, long-stemmed grasses sugar cane and corn can be used as typical
examples of grasses. Structures such as ligules and axillary buds are large enough in
these plants to be readily seen, and at the same time they are two important crop plants
which deserve attention in this course.
Sugarcane (Sacharrum officinarum)
Leaves
Examine the leaves and how they are arranged on the stem. Do they have ligules?
Auricles? What is their phyllotaxy? Make sure that you can identify these features.
Stem internodes
Examine the cane stem (culm) and draw an internode with the nodes at each end to
show root primordia, wax bands, axillary buds, leaf scars, and other features observed.
Stem apex
Examine the demonstration slide of a stem apex which has been cut in two
longitudinally to show the apical dome covered with extending leaves, and the
progressively longer internodes. Remember that each leaf base wraps around the stem,
so you are seeing each leaf twice.
Corn
The corn plant has separate male (tassel) and female (ear) flowers. It is grown for the
contents of its seed rather than the contents of its stem, and tillering has been bred out,
hence it is different from the sugar cane plant. It does have typical grass features, like
strap-like leaves with sheathing bases (do they have auricles or ligules?), and
adventitious roots at the lower nodes (which form prop roots, rather than being
propagative).
Examine a plant which has been split in half longitudinally through an ear, and draw the
region of stem with the attached ear (which is growing from an axillary shoot) and
associated sheathing leaves. What is the phyllotaxy of the corn plant?
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Stem and Root Crops
You have already studied a good example of a stem storage organ, sugarcane (also
classed as an aerial stem tuber), and had a review of the types of stem crops and root
crops. Also, you learned about the secondary growth in trees that produces wood and
bark, and about the anomalous secondary growth patterns that produce many organs of
perinnation. Now you will see how storage organs form and study the morphology of
some stem/root crops.
Dasheen corm (Colocasia esculentem)
A corm is an upright, enlarged base of a stem with scale leaves. The rings on the dasheen
corm are nodes from which adventitious roots and the scale leaves grow. Foliage
leaves are produced at the top of the corm. Dasheen is a monocot like sugarcane, and
if you recall the stem morphology of cane, you will see that the dasheen corm is like a
drastically shortened length of cane with vestigial leaves. Axillary bud growth is strongly
suppressed in dasheen. Draw a corm showing the features mentioned.
Ginger rhizome (Zingiber officinale)
A rhizome is a horizontal underground storage stem with scale leaves. The ginger
rhizome is interesting because it has contractile roots that keep it buried in the soil,
away from herbivores. Draw a rhizome with its attached contractile roots.
Root tuber of sweet potato (Ipomoea batatas)
In some classifications, the root tuber of sweet potato is called a swollen adventitious
root. Draw an entire plant showing leaves, tubers, and attached roots. The somewhat
fibrous texture of the cooked sweet potato results from vascular bundles in it, and you
can see this vascular tissue by looking at young roots. If you examine the prepared slide
showing stem tuberization (the tuberization process occurs in both roots and stems), you
will see cells radiating from cambial tissue wrapped around vascular bundles, which are
mainly xylem vessels. Cambium does not always produce vascular tissue or bark. Here,
it is making secondary cortex, showing a form of anomalous secondary growth. Make
a tissue map of the stem tuber section on the slide to show the areas of anomalous
secondary growth.
Beetroot (Beta vulgaris)
Examine a prepared slide of a section of a root for anomalous secondary growth. You
will see rings or bands where secondary xylem and cortex are being produced from
cambial layers. Make a drawing of this anomalous secondary development.
öExamine and draw the cassava, toppee tambu plants, or other material supplied.
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Exercise 4
Flowers
Flowers exhibit a great variety of modifications, and these influence their reproductive
characteristics. While many flowers are small (notably the grasses), there are several
examples of relatively large flowered plants amongst common crops that represent a
number of important modifications. The specimens to be examined in this Exercise will
vary depending on availability. Examine and draw the flowers as instructed.
Monoecious and dioecious species
1. Members of the Cucurbitaceae (cucumber, watermelon, pumpkin, etc.,) have
unisexual flowers, which are borne singly in the axils of its leaves, and the plants are
either monoecious or dioecious.
2. The banana inflorescence, which is a spike, has male flowers at the tip and female
flowers at the base, with hermaphrodite flowers in between.
3. Coconut inflorescences are complex branched racemes, but like the banana their
flowers are unisexual, with male flowers toward the tip of each rachis and female
blooms toward the base.
Zygomorphic flowers
4. Characteristically the flowers of orchids and legumes have bilateral symmetry, which
evolved to facilitate pollination by flying insects. Orchid blooms have been further
modified by horticulturists, while legumes have been bred for better yield rather than
for showier flowers. Many wild orchids are small, not brilliantly coloured, and
inconspicuous.
Chasmogamous and cleistogamous flowers
5. Flowers that open partly or fully at the time of pollen shedding (dehiscence) are
termed chasmogamous. Outcrossing is promoted if a flower opens fully, but if the flower
opens only partly, shed pollen is prevented from escaping by the enclosing petals, e.g.,
in tomato. This encourages self pollination and discourages outcrossing. Cleistogamous
flowers remain closed during dehiscence, essentially eliminating outcrossing.
Flowers with false stamens
6. Cocoa blooms, like tomato, are chasmogamous. The pistils are enclosed by curved
petals but tiny midges, are enticed by redbrown false stamens (staminodes) into the
flower. To escape they must crawl over the anthers and the stigmas, effecting pollination
of the same flower and pollinating other flowers if they carry pollen to them.
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Floral structure can be represented in a number of ways designed to standardize their
seemingly complex form.
Floral Formula
The floral formula uses a set of numbers, letters, and symbols, (not officially
standardized), to represent the floral parts, their number, and how they are fused.
The whorls are designated: E: epicalyx, K: calyx, C: corolla: P: perianth, A; androecium,
and G: gynaecium.
The number of parts in each whorl is written after the appropriate letter.
Fusion between parts in a whorl is indicated by enclosing the number in parentheses,
e.g., (5)
Fusion between whorls is shown by linking the letters by a curved line, K C.
A superior ovary is shown by underlining the number of carpels, 5.
An inferior ovary is shown with a line above the number of carpels, 5.
Zygomorphic flowers are represented by ^placed before the formula.
Actinomorphic flowers are indicated by +.
Half Flower Drawing
This is a drawing of a vertical section of the flower made through the line of symmetry
(back to front).
Floral Diagram
The floral diagram is a cross section (transverse section) of the flower, including the
ovary to show the carpels and placentation. Fusion of parts is shown by joining them
with a loop or line. Care must be taken to show aestivation (overlapping) of parts.
Ipomoea tiliacae (wild slip)
Psidium guajava (guava)
+K5C(5)A5G(2)
+K (4-6)C
A4G(5)
4-6
© 2006 G. F. Barclay - Plants Alive
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Exercise 5
Fruit Structure and Classification
Fruits are, to a botanist, different from what may be construed from the popular notion
of "fruits and vegetables." The green bean (Phaseolus vulgaris), also called the salad
bean, is a fruit, not a "vegetable," the coconut is not a nut (although both coconuts and
nuts are fruits), and the banana is an example of a berry.
A true fruit is the mature ovary of an angiosperm. The maturation of the ovule(s) to
form the seed(s) within the ovary is normally accompanied by a thickening of the ovary
wall to form three layers that comprise the pericarp. The outer layer (exocarp) may
simply be a layer of epidermis, as in the grape, the middle layer (mesocarp) is
commonly soft, like the flesh of the mango, and the inner layer (endocarp) can vary
from gelatinous (tomato) to stony (the "pit" of the plum).
In general, fruits may be classified according to four criteria:
e the number of flowers involved in their formation.
e the number of ovaries.
e the degree of hardness of the mesocarp - dry and hard or soft and fleshy.
e the ability of the fruit to dehisce (split open when mature) or not.
Examine and draw the examples of the fruit types provided. Make sure you can
identify each type based on the characteristics it displays.
A Classification of Fruits
1. Simple Fruits
Fruit derived from a solitary carpel in a single flower
A. Dry Fruits
Having a mesocarp that is definitely dry at maturity
(i) Indehiscent Fruits
(a) Developing from a single carpel
1. Achene - simple and small. Fruit wall usually thin and papery, with seed loose inside
(Helianthus).
2. Caryopsis - also called grain - like an achene, but pericarp and testa are fused (all
grasses).
3. Samara - one-seeded fruit with wing-like extensions (elm, Ulmus americana;
Myroxylon sp.).
© 2006 G. F. Barclay - Plants Alive
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(b) Developing from a compound gynoecium with several carpels
4. Nut - ovary with several carpels; all but one degenerate. Mature nut has one seed, and
a woody pericarp of sclereids (hazelnut).
(ii) Dehiscent Fruits
(a) Developing from a single carpel
5. Follicle - pod-like fruit, splits open on one side (milkweed, columbine).
6. Legume - breaks open on both sides (beans, peas).
7. Silique - two sides split, seeds remain attached to a false central partition or replum
(Brassicaceae) The silique is classified as a capsule by some taxonomists.
(b) Developing from a compound gynoecium with several carpels
8. Capsule - carpels may split in various ways (mahogany, cotton, iris, lily, poppy; akee,
Blighia sapida, may be considered a fleshy capsule).
(iii) Schizocarpic Fruits
Arising from a two or more- locular compound ovary; the locules split into separate (usu.
dry) fruits
9. Schizocarpic mericarp - twin mericarpic fruits joined at one point (parsley family)
B. Fleshy Fruits
Having a mesocarp that is soft at maturity
10. Berry - (true berry or bacca) - endocarp, mesocarp, and exocarp are all rather soft
but easily distinguishable (grape, tomato)
11. Drupe - like a berry, but endocarp is thick and hard (mango, peach, almond).
12. Pome - (also classed as a false fruit) - like a drupe, but endocarp is papery, not stony
(apple).
13. Pepo - like a berry, but exocarp is hard, forming a rind (pumpkin, squash).
14. Hesperidium - leathery exocarp with oil glands (all citrus).
2. Aggregate Fruits
Formed from one flower with many ovaries maturing together, some types fusing with
receptacle
15. Etaerio - consists of an aggregate of achenes, berries, or drupes (Frageria, strawberry
- also classed as a pseudocarp, a kind of false fruit; Annonas muricata, soursop).
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3. Multiple Fruits
Developing from the ovaries of several flowers of an inflorescence which mature together
(i) Fleshy Multiple Fruits
(may also be classed as accessory fruits)
16. Sorosis - fruits on a common axis that are usually coalesced and derived from the
ovaries of several flowers (pineapple, Ananas comosus; breadnut, Artocarpus).
17. Syconium - a syncarp with achenes attached to the inside of an infolded receptacle
(Ficus).
(ii) Dry Multiple Fruits
18. Strobilus - multiple fruit of achenes incorporating bracts (hop, Humulus lupulus).
4. Accessory / False Fruits
Many tissues near the ovary wall contribute to the protection of the seeds. Examples of
these accessory tissues include: the receptacle of strawberry and soursop, the perianth
of breadnut, the pedicel of Anacardium (cashew "berry"), and the scaly bracts of
inflorescences (pineapple). The true fruits of a strawberry are the tiny yellow or brown
"seeds," which are really achenes, on the surface of the swollen red receptacle.
Any fruit that develops from an inferior ovary will be a false fruit. In an apple (and all
related fruits) the mesocarp merges into the receptacle with no exocarp visible. There
is no sign of a double structure. The carpels are perfectly "fused" to the accessory tissue
from primordial initiation (but note that two sets of vascular bundles, one previously
leading to sepals, the other to petals before the apple formed, remain visible in the
mature apple.) Therefore, it is not useful to distinguish the inner true fruit from the
outer false fruit.
© 2006 G. F. Barclay - Plants Alive
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Appendix
Confusing Terms
e Epidermis is covered with cutin, not chitin.
e Tree trunks are covered with periderm, not pericarp.
e Periderm has 3 layers:
] DO
phelloderm, phellogen, & phellem. ] NOT
e Pericarp also has 3 layers:
] CONFUSE
endocarp, mesocarp, & exocarp.
] THESE
e Protoderm, not periderm, gives rise to epidermis.
e Xylem vessels and tracheids have pits, not piths. Pits may be bordered pits, not
boarded pits.
e Collenchyma, not chlorenchyma, and sclerenchyma, not schlerenchyma, are support
tissues.
e In botany, coconuts are not nuts, they are drupes.
e Secondary growth refers to cambial activity, not secondary wall formation.
e Bark does not help support a tree. It inhibits desiccation and pathogen/insect attack.
e Plasmodesmata connect living cells. Pits, not piths, connect dead cells.
e Axillary buds, not auxiliary buds, arise in the axils of leaves.
e Don't say "cambium" if you want to avoid confusion amongst the cork, pro-, and
vascular types. Specify the correct form. Procambium produces 1° xylem and phloem.
Vascular cambium produces 2° xylem and phloem. Secondary growth of vascular tissue
results initially from the activity of the intra- and inter- fascicular cambia, which together
give rise to vascular cambium.
e The word is phloem, not pholem, and phloem has no vessels. "Phloem vessels" do not
exist.
e Do not confuse tissue with cell. Phloem and xylem are types of tissue, not types of
cell. Do not say phloem if you mean sieve tube, or xylem if you mean vessel. Xylem is
not dead. Its tracheids, vessels, and fibers are dead, but not its parenchyma, so it is
wrong to say "xylem is dead." Another example: the only mature, living plant cell to lack
a nucleus is the sieve element, not phloem. Remember the difference between cell and
tissue when considering periderm, and other complex tissues.
© 2006 G. F. Barclay - Plants Alive
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BIOL1764/BL11F (Anatomy/Morphology) Course Objectives
1. Describe the cells and tissues that comprise the support tissues and the vascular tissues, their
related functions, and their distribution in the plant.
2. Describe: meristems, cambium, primary & secondary growth.
3. Compare & contrast: xylem & phloem, sclerenchyma & collenchyma, primary & secondary
wall, pits & plasmodesmata, primary & secondary growth.
4. Describe phyllotaxy, stem branching, root branching.
5. Describe: epidermis and its modifications, periderm.
6. Compare & contrast: stoma & lenticel, shoot epidermis & root epidermis.
7. Describe the special anatomical/morphological features of a typical dicot leaf, and the
functional significance of these features.
8. Describe: the structural & related functional modifications of leaves.
9. Describe the external root zones and root structure, monocot/dicot root structure, root
modifications, organs of perinnation, quiescent center, pericycle, casparian band. Compare &
contrast: exodermis & endodermis
10. Compare & contrast: spines & tendrils, sclerophyllous & succulent leaves, spines & bud scales, etc.
11. Describe: heteroblasty, trap leaves, monocot/dicot stomatal structure in relation to function,
advantages of compound vs. simple leaves.
12. Describe: grass plant morphology, the morphological features of a typical grass leaf, the
sugarcane internode, tillering, booting, and the purpose of growth scales for cereals.
13. Describe: reaction wood, diffuse porous/ring porous wood, sapwood/heartwood, tyloses, rays,
hardwood/softwood.
14. Describe how secondary growth in storage roots/stems differs from ordinary secondary growth.
15. Describe floral structure, & describe its modifications resulting from ovary position, loss of parts,
and floral shape.
16. Describe how floral modifications influence pollination, fruit structure, and seed dispersal.
Describe how grass flowers differ from typical dicot flowers.
17. Describe the criteria for classifying fruit, and be able to identify fruit types. Describe how
pericarp structure affects seed dispersal.
18. Compare and contrast: androecium & gynoecium, staminate & pistillate flowers, pericarp &
periderm, true fruit & false fruit, multiple fruit & aggregate fruit.
Write concise notes on each of the organ modifications and on each of the Compare & Contrast terms.
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© 2006 G. F. Barclay - Plants Alive
List of Important Terms to Know
accessory fruit
achene
actinomorphic flower
adventitious branching
adventitious root
aerenchyma
aggregate fruit
alternate phyllotaxy
amyloplast
anomalous 2° growth
apex
apical meristem
aril
astrosclereid
auricle
axillary bud
axillary meristem
berry
boot/booting
bordered pit
bud scale
bulb
bulliform cell
bundle cap
bundle sheath
calyptrogen
cambium
capsule
carpel
caryopsis
casparian band
cataphyll
cellulose
collenchyma
coleorrhiza
complete flower
compression wood
companion cell
compound fruit
conjunctive tissue
contractile root
cork cambium
corm
cortex
cotyledon
cuticle
culm
cymose inflorescence
decussate phyllotaxy
dehiscent fruit
dewlap
dichasial branching
diffuse porous wood
distichous phyllotaxy
drupe
dry fruit
duct
endocarp
endodermis
endosperm
epidermis
epicuticular wax
exocarp
exodermis
false fruit
fibre
flag leaf
fleshy fruit
floral bract
follicle
guard cell
hardwood
heartwood
helical wall thickening
hesperidium
heteroblasty
hydathode
hydrophytic leaf
hypodermis
imperfect flower
incomplete flower
indehiscent fruit
inferior ovary
integument
intercalary mer.
interfascicular camb.
intrafascicular cambium
internode
Krantz anatomy
leaf lamina
lateral meristem
lateral root
laticifer
leaf axil
leaf primordium
leaf sheath
legume
lenticel
lignin
ligule
locule
macrosclereid
margo
meristem
mesocarp
mesophytic leaf
metaphloem, metaxylem
middle lamella
monopodial branching
mucigel
multiple fruit
nectary
node
nut
opposite phyllotaxy
organ of perennation
osteosclereid
ovule
parenchyma
palisade mesophyll
pepo
perfect flower
perforation plate
pericarp
pericycle
periderm
petiole
phellem
phelloderm
phellogen
phi thickening
pit
pit aperture
pith
phloem
© 2006 G. F. Barclay - Plants Alive
phyllotaxy
pistillate flower
plasmalemma
plasmodesma
polyarch xylem
pome
primary growth
primary meristem
primary wall
procambium
promeristem
prop root
protoderm
protophloem, protoxylem
pseudostem
quiescent center
racemose inflorescence
ratooning
ray
reaction wood
receptacle
reduced leaf
reticulate thickening
rhizome
ring porous wood
root cap
root hair, r. h. zone
root primordium
samara
sapwood
scalariform thickening
scale leaf
sclereid
sclerophyllous leaf
sclerenchyma
scutellum
secondary cortex
secondary growth
secondary meristem
secondary wall
sett
sieve element, tube
sieve plate, pore
silique
softwood
spine
spiral phyllotaxy
staminate flower
stele
stoma
storage root
suberin
succulent leaf
superior ovary
sympodial branching
tendril
tension wood
testa
tetrarch xylem
tiller, tillering
torus
tracheary element
tracheid
trichome
tuber
tylose
vascular cambium
velamen
vessel
vessel element
whorled phyllotaxy
xerophytic leaf
xylem
Numerical growth scales
for cereals
zone of elongation
zone of maturation
zygomorphic flower
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