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Chapter 31
PLANT FORM AND FUNCTION
There are about 262,000 species of plants. About 235,000 species or 90%, are angiosperms.
Angiosperms can be either herbaceous or woody.
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Herbaceous plants can be annuals, biennials and perennials.
Woody plants are perennials.
The plant body consists of a root system and a shoot system.
I. ROOT AND STEM SYSTEMS
DIVERSITY OF ROOTS
Roots show great diversity of structure but their function is similar.
Principal function of roots: support, storage and absorption.
Roots absorb many nutrients from the soil.
N, K and P are the limiting nutrients that plants need and often are depleted from the soil.
Roots may vary from long, deep, taproots to shallow, fibrous masses of roots. Adventitious roots originate
in organs that are not the primary root, e.g. leaves, branches.
DIVERSITY OF STEMS
Stems show great variety of structures: trunks, creeping stems, rosettes, tubers, rhizomes, etc.
Principal function of stems:
1. Support of the photosynthetic, reproductive and storage parts.
2. Conduction of water and metabolites.
3. Production of new stem tissues.
Different structure (shape, size, etc.) of stems and roots help plants survive in specific habitats.
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Stem variation is structure is adaptive.
Closely related species and families of plants tend to have similar morphology and anatomy inherited
from a common ancestor.
II. CELLS, TISSUES, ORGANS AND SYSTEMS.
Plants are made of cells organized into tissues and organs.
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Roots, stems, flowers and fruits are organs.
There are three tissue systems that extend throughout the entire body of the plant.
Each tissue system contains two or more tissues, which can be simple or complex depending on the
kinds of cells that form the tissue.
CELLS
Parenchyma cells
 Living cells at maturity.
 Have thin primary walls.
 Function in storage, secretion and photosynthesis.
 Found throughout the body of the plant.
Collenchyma cells
 Living cells at maturity.
 Have unevenly thickened primary cell walls.
 Function in support in flexible parts of the plant.
 Found in petioles, leaf veins and other parts of the plant that must be flexible.
Sclerenchyma cells
 Have both primary and thickened secondary cell walls.
 Cells are often dead at maturity.
 Secondary wall with pits.
 Provide structural support.
Tracheids
 Dead at maturity.
 Long and tapered cells.
 Have secondary walls except at pits.
 Pits on lateral and end walls.
 Conduction of water and minerals.
Vessel elements
 Dead at maturity.
 Long and cylindrical cells joined end to end.
 Secondary cell wall except at pits.
 End walls have perforations.
 Conduction of water and minerals.
Sieve tube members
 Living cells at maturity.
 Lack nucleus and other organelles at maturity.
 Elongated cells, cylindrical, joined end to end.
 Secondary cell wall present.
 End walls are sieve plates with holes.
 Cytoplasm extends from one cell to the next through the holes of the sieve plate.
 Conduct the products of photosynthesis.
Companion cells
 Living cells at maturity.
 Associated to a sieve tube members by means of plasmodesmata.
 Assists in moving sugars in and out of sieve tube members.
 The nucleus is thought to direct the activity of both cells.
TISSUES
1. Epidermal tissue consists of a single layer of cells, the epidermis, which covers the entire body of
the plant except the woody areas where cork tissue develops.
2. Cork tissue or periderm consists of cork cells, cork cambium and cork parenchyma.
3. Ground tissue is located below the epidermis and surrounds the vascular tissue. It is made up of
parenchyma cells often strengthened with collenchyma and sclerenchyma cells.
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Photosynthesis, storage, secretion, flexible and rigid structural support.
4. Vascular tissue is complex. It is made of two tissues:
Xylem
tracheids
vessel elements
parenchyma cells
fibers
Phloem
sieve tube members
companion cells
parenchyma cells
fibers
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The function of the xylem is to transport water and nutrients from the roots to other parts of
the plant.
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The function of the phloem is transport photosynthates from the green areas of the plant to
the living cells found in the roots, stems, etc.
III. ANATOMY.
Growth in plants is localized in regions called meristems.
ROOT ANATOMY
The root has three concentric tissues: epidermal, ground and vascular tissues.
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The vascular tissue forms a central cylinder called the stele.
Secondary roots arise from a layer of cells in the stele called the pericycle, and grow horizontally
until they erupt through the epidermis.
The epidermis has root hairs used in absorption.
Root structure is different from that of the stem. It differs in.
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Root cap, a protective layer that covers the apical meristem and orients the root to grow
downward. Root cap responds to gravity (gravitropism).
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Root hairs, short-lived extensions of the epidermal layer, to increase absorption.
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Roots have a pericycle and endodermis present.
Roots lack nodes and internodes.
STEM ANATOMY
External morphology
1. Terminal and lateral buds.
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Covered with bud scales while dormant.
Contain a growth cluster of cells called meristem and produce new or primary tissues.
Lateral buds are associated with leaf axils.
Bud scale scars.
2. Nodes and internodes.
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Nodes are the regions of leaf attachment.
Internodes are the space between two nodes.
3. Leaf scars and bundle scars.
4. Lenticels.
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Loosely arranged cells that allow gas exchange.
Broken epidermis.
LEAF ANATOMY
External morphology
Leaves are organs and vary greatly in external form.
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They may range in length from about 20 m (65 ft) to about 0.15 cm (0.06 in).
Their principal parts are blade, petiole and stipules (may be absent).
Leaves can be simple or compound.
Their arrangement along the stem can be alternate, opposite or whorled.
Their venation may be netted or parallel.
Veins can be arranged pinnately or palmately
Anatomy
1. Epidermis
There are upper and lower epidermises that form the surface of the leaf.
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Made of living parenchyma cells.
Lack chloroplasts.
Covered with a waxy layer, the cuticle.
Have small openings for gas exchange called stomata (sing. stoma).
Guard cells flank each stoma.
Trichomes or hairs may be present.
2. Mesophyll
The photosynthetic tissue found in between the two epidermises is called mesophyll.
It consist of...
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Made of living parenchyma cells.
Abundant chloroplasts.
Usually loosely arranged with many air spaces.
Often arrange in two regions: palisade and spongy mesophyll.
3. Venation
Veins and diffusion cooperate in the movement of materials in veins.
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Veins or vascular bundles extend through the mesophyll.
Each vein contains xylem and phloem tissue.
Xylem is usually restricted to the upper side of the vein, and phloem to the lower side of the vein.
A non-vascular parenchymatous layer of cells called the bundle sheath surrounds veins.
The bundle sheath extensions are support columns of cells that extend from the vein to the
upper and lower epidermis.
May be composed of parenchyma, collenchyma or sclerenchyma cells.
Monocot leaves usually have parallel venation and it is not differentiated into palisade and spongy
mesophyll; the guard cells are shaped like dumbbells
Dicot leaves have netted venation, mesophyll differentiated into two regions, and the guard cells are
bean-shaped.
Subsidiary cells are epidermal cells associated with the stomata and are involved in the opening and
closing of the stomata.
Gymnosperms normally have parallel or free venation (not netted).
GROWTH
Growth in plants is localized in regions called meristems.
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It involves cell division, cell elongation and cell differentiation.
1. Primary growth causes the roots and stems to elongate.
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It occurs in all plants.
The apical meristem at the tip of roots and stems is responsible.
2. Secondary growth is an increase in stem and root girth.
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It occurs in woody plants and a few herbaceous plants only.
It is due to the activity of the lateral meristems: vascular cambium and cork cambium.
Vascular cambium forms a cylinder along the length of roots and stems, between the xylem and
phloem; it produces more xylem and phloem.
Cork cambium is located in the outer bark.
Tree rings are formed by the alternation of periods of growth and dormancy.
Genetically identical individuals differ in response to environmental differences (e.g. growing in the shade
or in the sun). This is called developmental plasticity.