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Plant Structure, Growth, and Development
Overview: No Two Plants Are Alike
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The form of any plant is controlled by environmental and
genetic factors. As a result, no two plants are identical.
Plants have specialized organs that allow them to obtain
water and nutrients from the soil and to absorb water and
CO2 from the air.
Plant species have adaptive features that benefit them in
their specific environments.
Can you think of some adaptations of plants and their
function?
Plants Structure
Plants, like multicellular animals, have organs that are composed of
different tissues, and tissues are composed of different cell types.
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A tissue is a group of cells with a common structure
and function.
An organ consists of several types of tissues that
work together to carry out particular functions. Plant
organs include roots, stems, leaves and flowers.
Plant Tissues: Each organ of a plant has three tissue systems
that are continuous throughout the plant: dermal, vascular, and
ground.
1. Dermal tissue is the outer covering.
In nonwoody plants, it is a single layer of tightly packed cells,
or epidermis, that covers and protects all young parts of the
plant.
Root hairs are extensions of epidermal cells near the
tips of the roots.
The epidermis of leaves and most stems secretes a
waxy coating, the cuticle, which helps the aerial parts
of the plant retain water.
2.
Vascular tissue, continuous throughout the plant, is involved
in the transport of materials between roots and
shoots.
There are two main types of vascular tissue
1. Xylem moves water and dissolved minerals upward
from roots into the shoots.
2. Phloem transports food made in mature leaves to the
roots; to nonphotosynthetic parts of the shoot
system; and to sites of growth, such as developing
leaves and fruits.
3. Ground tissue is tissue that is not dermal tissue or
vascular tissue.
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Functions of ground tissue include photosynthesis,
storage, and support.
Plant Cell Types: Plant tissues are composed of three basic cell
types: parenchyma, collenchyma, and sclerenchyma.
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After formation, many plant cells become differentiated,
meaning that each cell grows and develops structural
adaptations that allow it to do a specific job in the cell.
The major types of differentiated plant cells include:
parenchyma, collenchyma, sclerenchyma, water-conducting
cells of the xylem and sugar-conducting cells of the phloem.
1. Parenchyma cells:
* Mature parenchyma cells have primary walls that
are relatively thin and flexible, and most lack
secondary walls.
* They usually have a large central vacuole.
* Parenchyma cells are often depicted as “typical”
plant cells because they generally are the least
specialized. There are exceptions to this
generalization, for example, the highly
specialized sieve-tubes of the phloem are
parenchyma cells.
* Parenchyma cells perform most of the metabolic
functions of the plant
- parenchyma cells in the leaf photosynthesize
- parenchyma cells in stems and roots store
starch
- the fleshy tissue of most fruit is parenchyma
tissue
* Most parenchyma cells retain the ability to divide
and differentiate into other cell types under
special conditions, such as the repair and
replacement of organs after injury to the plant.
In the laboratory, it is possible to regenerate an
entire plant from a single parenchyma cell.
2. Collenchyma cells have thicker primary walls than parenchyma
cells, though the walls are unevenly thickened.
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They are often grouped into strands or cylinders, and are
located just below the epidermis and provide support.
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Mature collenchyma cells are living and flexible and elongate
with the stems and leaves they support.
3. Sclerenchyma cells have thick secondary walls usually
strengthened by lignin and function as supporting elements
of the plant.
 They are much more rigid than collenchyma cells.
 Unlike parenchyma and collenchyma cells, they cannot
elongate, so sclerenchyma cells occur in plant regions that
have stopped lengthening.
 Many sclerenchyma cells are so specialized that they are
dead at functional maturity, having produced the secondary
walls before the protoplast died.
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Two types of sclerenchyma cells, fibers and sclereids, are
specialized entirely for support.
Fibers are long, slender, and tapered, and usually occur
in groups. Examples include hemp used to make
rope and flax used to make linen cloth
Sclereids are irregular in shape and shorter than fibers.
They have very thick, lignified secondary walls.
Sclereids impart hardness to nutshells and seed
coats and the gritty texture to pear fruits.
4. Xylem: Xylem is composed of various types of cells:
parenchyma, fibers, and special water-conducting cells of
xylem called tracheids and vessel elements.
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Tracheids and vessel elements are elongated cells that are
dead at functional maturity.
The thickened secondary cell walls remain as a
nonliving conduit through which water can flow.
Tracheids are long, thin cells with tapered ends
 Water moves from cell to cell mainly through pits: thin-walled
areas between cells through which water and dissolved
substances can flow.
 Because their secondary walls are hardened with lignin,
tracheids function in support as well as transport.
Vessel elements are generally wider, shorter, thinner walled, and
less tapered than tracheids.
Xylem tracheids and vessels elements
Phloem sieve tubes and
Companion cells
5. Phloem moves sucrose, other organic compounds, and some
mineral ions move through tubes formed by chains of cells
called sieve-tube members.
 Phloem tissue includes parenchyma, schlerenchyma, sieve
tubes and companion cells.
 Phloem cells are alive at functional maturity, although a
sieve-tube cell lacks a nucleus, ribosomes, and a distinct
vacuole.
 Adjoining end walls, contain sieve plates which have pores
that facilitate the flow of fluid between cells.
 Each sieve-tube member has a nonconducting nucleated
companion cell, which is connected to the sieve-tube
member by numerous plasmodesmata.
The nucleus and ribosomes of the companion cell serve
both that cell and the adjacent sieve-tube member.
In some plants, companion cells actively help load
sugar into the sieve-tube members, which transport the
sugars to other parts of the plant.
Vascular plants have three basic organs: roots, stems, and
leaves.
The basic morphology of vascular plants reflects their
evolutionary history as terrestrial organisms that inhabit and
draw resources from two very different environments:
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They obtain water and minerals from the soil.
They obtain CO2 and light above ground.
To obtain the resources they need, vascular plants have
evolved two systems that must work together:
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a subterranean root system that absorbs water
and minerals for the entire plant
an aerial shoot system of stems and leaves
that photosynthesize to provide sugar for
the entire plant.
ROOTS:
A root is an organ that functions to:
a. anchors a vascular plant in the soil,
b. absorbs minerals and water, and stores food.
c. Store food (in many plants)
Absorption of water and minerals occurs near the root
tips, where vast numbers of tiny root hairs enormously
increase the surface area.
 Root hairs are extensions of individual epidermal
cells on the root surface.
 Absorption of water and minerals is also increased
by mutualistic relationships between plant roots
and fungi (Mycorrhizae) or by plant roots and
bacteria (Ex Nitrogen fixing bacteria)
Most eudicots and gymnosperms have a taproot system,
consisting of one large vertical root (the taproot) that
produces many small lateral, or branch, roots.
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taproots often store food that supports flowering
and fruit production later.
Seedless vascular plants and most monocots, including grasses,
have fibrous root systems consisting of a mat of thin roots
that spread out below the soil surface.
 A fibrous root system is usually shallower than a
taproot system.
 Grass roots are concentrated in the upper few
centimeters of soil. As a result, grasses make
excellent ground cover for preventing erosion.
Anatomy of a root:
Root Cap: The tip of the root is covered by a cap that is shaped
like a thimble. The outer cells of the root cap are
continuously being worn away and new cells are added to
the inner portion. As these cells disintegrate they form a
strong protective cover.
Zone of Cell Division: A zone of active cell division (called an
apical meristem) which rapidly forms new cells
which differentiate later to form more specialized root
tissues. The cells of this region also replace the cells
rubbed-off from the root cap
Zone of Elongation: Cells in this zone undergo rapid growth in
length and cause the root to elongate and penetrate deeper
into the soil.
Epidermis: of the root is dermal tissue and is made up of thin
walled parenchyma cells with no cuticle.
 Its function is to protect the root.
Cortex: the cortex is a fairly wide region of ground tissue that is
active in the uptake of water and minerals and also
functions to store starch and other molecules.
Stele: the bundle of vascular tissue (xylem and phloem)
Endodermis: a cylinder of cells one cell thick that forms a
boundary between the cortex and the stele
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It contains the casparian strip, a cell wall layer
containing suberin and sometimes lignin on the
endodermis cells that is waterproof and prevents
water entering the vascular tissue without first
moving through the plasma membrane and into a
cell.
Pericycle: a layer of cells found just inside of the endodermis, it
may become meristematic and result in the formation
of lateral roots
Root anatomy of eudicots:
 Xylem and phloem in groups of 6-8
 Xylem forms and “X” shape in stele (center)
 Pith is absent or small (Pith is found in the very
central region of some roots and stems, and is
made up of parenchyma cells).
 Xylem elements are angular or polygonal in
transverse section.
Root anatomy in monocots:
 Xylem and phloem in many alternating bundles
forming circle around the root.
 Pith is large and well developed
Modified Roots:
Some arise from roots while adventitious roots arise
Above ground from stems or even from leaves.
Some modified roots provide additional support and
anchorage. Others store water and nutrients or
absorb oxygen or water from the air
Storage roots (sweet potato)
Air roots (mangrove) enable plants to get extra oxygen in
water logged soil
Mangrove tree
STEMS: A stem is an organ that raises or separates leaves,
exposing them to sunlight.
A stem consists of :
 alternating nodes, the points at which leaves are
attached,
 internodes, the stem segments between nodes.
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At the angle formed by each leaf and the stem is
an axillary bud with the potential to form a lateral
shoot or branch.
Growth of a young shoot is usually concentrated at its apex,
(tip) where there is a apical meristem , or an area of
sustained cell division. Cells differentiate to form
vascular tissue, leaves, stem cells. As the cells
mature, they elongate and cause the stem to
elongate.
The terminal bud produces hormones that travel down the
stem and inhibiting the growth of axillary buds, a
phenomenon called apical dominance.
By concentrating resources on growing taller, apical
dominance is an evolutionary adaptation that
increases the plant’s exposure to light.
In the absence of a terminal bud, the axillary buds
break dominance and give rise to a vegetative branch
complete with its own terminal bud, leaves, and
axillary buds.
By pruning or pinching the apical meristem (bud) off of a plant, you can
encourage the axillary buds to grow and cause branching.
Monocot and herbaceous Eudicot (Dicot) stems vary in the
arrangement of their vascular tissue.
Pith
In eudicot stems, ground tissue is divided into pith , located inside
the vascular tissue, and cortex, found outside the vascular tissue
Modified shoots with diverse functions have evolved in many
plants.
Stolons, such as the “runners” of strawberry plants, are
horizontal stems that grow on the surface and
enable a plant to reproduce asexually as
plantlets form at tips of each runner.
Rhizomes, Sturdy, horizontal, underground stems called
rhizomes anchor large monocots such as palms and
bamboo. Another example is ginger.
Tubers, including potatoes, are the swollen ends of
rhizomes specialized for food storage.
Bulbs, such as onions, are vertical, underground shoots
consisting mostly of the swollen bases of leaves that
store food.
Tuber
Bamboo rhizome
Bulb
LEAVES
Leaves are the main photosynthetic organs of most plants,
although green stems are also photosynthetic.
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While leaves vary extensively in form, they generally
consist of a flattened blade and a stalk, called the
petiole, which joins the leaf to a stem node.
Grasses and other monocots lack petioles. In these
plants, the base of the leaf forms a sheath that
envelops the stem.
Most monocots have parallel major veins (bundles of
xylem & phloem) that run the length of the blade
Eudicot leaves have a multibranched network of
major veins.
Most leaves are specialized for photosynthesis.
Some plants have leaves that have become adapted for
other functions
- tendrils that cling to supports (peas, cucumbers)
- spines of cacti
- leaves modified for water storage (jade plant,
sedum)
Structure of a leaf:
Cuticle: waxy layer on top and bottom epidermis. Minimizes water
loss.
Vein: a bundle of xylem and phloem.
Palisades cells: tightly packed vertically arranged cells located
under the upper epidermis. They’re main function is
photosynthesis.
Spongy mesophyll: irregular shaped cells interspersed with many
air spaces that allow for gas (CO2) exchange. They usually
contain a few chloroplasts and carry out photosynthesis.
Stomata are tiny pores in the epidermis (typically the lower
epidermis). They consist of two specialized guard cells that
actively intake K+ ions, causing an intake of water that
swells (opens) the pore. The guard cells “deflate” or close
when they loose water, thus closing the opening in
the leaf.
Stomata function to control the exchange of gases (CO2 and
water vapor) in the leaf.
Determinate vs Indeterminate Growth
A major difference between plants and most animals is that
plant growth is not limited to an embryonic period.
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Most plants demonstrate indeterminate growth,
growing as long as the plant lives.
In contrast, most animals, some plants, and
certain plant organs, such as flowers and leaves,
undergo determinate growth, ceasing to grow after
they reach a certain size.
Annual – Biennial - Perennial
Annual plants complete their life cycle—from germination
through flowering and seed production to death—in a
single year or less.
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Many wildflowers and important food crops,
such as cereals and legumes, are annuals.
A biennial plant’s life spans two years. Biennials often grow
vegetatively the first year, the flower the second year,
often with a cold season in-between.
A perennial is a plants such as trees, shrubs, and some
grasses that live many years.
Perennials do not usually die from old age, but from
an infection or some environmental trauma.
A perennial is capable of indeterminate growth
because it has perpetually embryonic tissues called
meristems in its regions of growth.
These cells divide to generate additional cells, some
of which remain in the meristematic region, while
others become specialized and are incorporated into
the tissues and organs of the growing plant.
Primary Growth is the increase in shoot or root length through cell
division at apical meristems , located at the tips of roots and
in the buds of shoots, supply cells for the plant to grow in
length.
 In herbaceous plants, primary growth produces
almost all of the plant body. Herbaceous plants
die back every year when the weather gets cold.
Herbaceous plants do not have a woody stem that
stays above ground when the leaves are gone.
Secondary Growth in perennials:
Woody plants also show secondary growth, progressive thickening
of roots and shoots where primary growth has ceased.
Secondary growth is produced by lateral meristems,
cylinders of dividing cells that extend along the length of
roots and shoots.
1. The vascular cambium adds layers of
vascular tissue called secondary xylem and
phloem.
2. The cork cambium replaces the epidermis
with thicker, tougher periderm.(bark)
The vascular cambium is located just outside the primary xylem
and to the interior of the primary phloem.
 The cells of the vascular cambium divide and form
secondary xylem (tracheids and vessel elements)
to the inside and secondary phloem (sieve
elements and companion cells) to the outside.
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The thickening of the stem that occurs in
secondary growth is due to the formation of
secondary phloem and secondary xylem by the
vascular cambium, plus the action of cork
cambium which forms the bark.
 The cells of the secondary xylem contain lignin,
which provides hardiness and strength.
In woody plants, cork cambium is the outermost lateral meristem.
It produces cork cells (bark) containing a waxy substance
known as suberin that can repel water.
 The bark protects the plant against physical
damage and helps reduce water loss. The cork
cambium also produces a layer of cells known as
phelloderm, which grows inward from the
cambium. The cork cambium, cork cells, and
phelloderm are collectively termed the periderm.
The periderm substitutes for the epidermis in
mature plants.
Growth Rings: The activity of the vascular cambium gives rise to
annual growth rings.
 During the spring growing season, cells of the
secondary xylem have a large internal diameter;
their primary cell walls are not extensively
thickened. This is known as early wood, or spring
wood.
 During the fall season, the secondary xylem
develops thickened cell walls, forming late wood,
or autumn wood, which is denser than early wood.
 This alternation of early and late wood results in
the formation of an annual ring, which can be seen
as a circular ring in the cross section of the stem .
An examination of the number of annual rings and
their nature (such as their size and cell wall
thickness) can reveal the age of the tree and the
prevailing climatic conditions during each season.
Angiosperms produce reproductive structures called flowers
and fruit.
Flowers: shoots that have become specialized for sexual
reproduction.
The flower structure has up to four rings of modified leaves:
Starting at the base (stem end of the flower) these rings of
modified leaves are:
1. Sepals: usually green, they are the outer layer of
the bud and encase the flower before it opens.
2. Petals: generally brightly colored they function to
attract pollinators.
- wind pollinated flowers usually lack colorful
parts
The sepals and petals are sterile, meaning they
are not involved with the production of eggs and
sperm.
3. Stamen: located within the petals, stamen
produce microspores that develop into pollen
grains that contain the male gametophyte.
- have a stalk called the filament
- terminal end contains a sac called the anther
where the pollen is produced.
4. Carpel – located within the petals, the carpel
produces a single or multiple female
gametophytes.
- Stigma – the sticky tip that receives the pollen
- Style – connects the stigma to the ovary
- Ovary – located at the base of the carpel, it
contains one or more ovules
(containing the female gametophytes)
If fertilized, the ovule develops into a
seed.
Fruit usually consists of a mature ovary. As the seed develops, the
walls of the ovary thicken.
 The fruit protects the seeds and aids in the
dispersal of the seeds.
 Fruits may be fleshy (grape, tomato) or dry (bean,
nut, grain).