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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 1 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 © 2006 G. F. Barclay - Plants Alive 2 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. © 2006 G. F. Barclay - Plants Alive 3 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. © 2006 G. F. Barclay - Plants Alive 4 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 © 2006 G. F. Barclay - Plants Alive 5 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. © 2006 G. F. Barclay - Plants Alive 6 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. © 2006 G. F. Barclay - Plants Alive 7 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 8 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. © 2006 G. F. Barclay - Plants Alive 9 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. © 2006 G. F. Barclay - Plants Alive 10 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 © 2006 G. F. Barclay - Plants Alive 11 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 © 2006 G. F. Barclay - Plants Alive 12 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. © 2006 G. F. Barclay - Plants Alive 13 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. © 2006 G. F. Barclay - Plants Alive 15 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. © 2006 G. F. Barclay - Plants Alive 16 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. © 2006 G. F. Barclay - Plants Alive 17 [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. © 2006 G. F. Barclay - Plants Alive 18 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. © 2006 G. F. Barclay - Plants Alive 19 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 20 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 © 2006 G. F. Barclay - Plants Alive 21 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. © 2006 G. F. Barclay - Plants Alive 22 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). © 2006 G. F. Barclay - Plants Alive 23 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 © 2006 G. F. Barclay - Plants Alive 24 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) © 2006 G. F. Barclay - Plants Alive 25 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. © 2006 G. F. Barclay - Plants Alive 26 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. © 2006 G. F. Barclay - Plants Alive 27 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 © 2006 G. F. Barclay - Plants Alive 28 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. © 2006 G. F. Barclay - Plants Alive 29 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: © 2006 G. F. Barclay - Plants Alive 30 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 © 2006 G. F. Barclay - Plants Alive 31 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). © 2006 G. F. Barclay - Plants Alive 32 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. © 2006 G. F. Barclay - Plants Alive 33 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. © 2006 G. F. Barclay - Plants Alive 34 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. © 2006 G. F. Barclay - Plants Alive 35 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. © 2006 G. F. Barclay - Plants Alive 36 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 © 2006 G. F. Barclay - Plants Alive 37 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. © 2006 G. F. Barclay - Plants Alive 38 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. © 2006 G. F. Barclay - Plants Alive 39 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. © 2006 G. F. Barclay - Plants Alive 40 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 © 2006 G. F. Barclay - Plants Alive 41 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 © 2006 G. F. Barclay - Plants Alive 42 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. © 2006 G. F. Barclay - Plants Alive 43 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 © 2006 G. F. Barclay - Plants Alive 44 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 45 © 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 © 2006 G. F. Barclay - Plants Alive 46 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. 47 © 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 48 © 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? © 2006 G. F. Barclay - Plants Alive 49 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. © 2006 G. F. Barclay - Plants Alive 50 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? © 2006 G. F. Barclay - Plants Alive 51 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. © 2006 G. F. Barclay - Plants Alive 52 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. 53 © 2006 G. F. Barclay - Plants Alive 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 54 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 55 (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). © 2006 G. F. Barclay - Plants Alive 56 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 57 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 58 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. 59 © 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 60