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Chapter 31
PLANT STRUCTURE, GROWTH AND
DIFFERENTIATION
There are about 262,000 species of plants. About 235,000 species or 90%, are angiosperms.
Angiosperms can be either herbaceous or woody.
Herbaceous plants can be annuals, biennials and perennials.
Woody plants are perennials.
THE PLANT BODY
The plant body consists of a root system and a shoot system.
The shoot system consists of a stem that bears leaves, flowers, fruits and sometimes adventitious roots.
Plants are made of cells organized into tissues and organs.
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.
A growing plant cell secretes a primary cell wall that stretches as the cell increases in size.
The secondary cell wall is deposited after the cell stops growing.
CELL TYPES
Parenchyma cells
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Living cells at maturity.
Have thin primary walls.
Function in storage, secretion and photosynthesis.
Found throughout the body of the plant.
Collenchyma cells
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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
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Have both primary and thickened secondary cell walls.
Cells are often dead at maturity.
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Secondary wall with pits.
Provide structural support.
There are two types of sclerenchyma cells: sclereids and fibers.
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Sclereids may be living or dead at maturity.
Short, cubical cells.
Sclereid rich tissue may be hard and inflexible.
Form part of the shell and pits of fruits, e.g. coconuts, walnuts, cherries, etc.
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Fibers are often dead at maturity.
Long, tapered cells often in clumps.
Have few pits in their secondary wall.
Provide strength and elasticity.
Found throughout the plant body, common in stems and some leaves.
Tracheids
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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
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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
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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
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Living cells at maturity.
Associated to a sieve tube member 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. GROUND TISSUE SYSTEM
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Composed of three simple tissues.
Parenchyma tissue
Collenchyma tissue
Sclerenchyma tissue
parenchyma cells
Collenchyma cells
sclereids, fibers
2. VASCULAR TISSUE SYSTEM
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Composed of two complex tissues.
Xylem
Tracheids
Vessel elements
Parenchyma cells
Fibers
Phloem
Sieve tube members
Companion cells
Parenchyma cells
Fibers
3. DERMAL TISSUE SYSTEM
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Composed of two complex tissues.
Epidermis
Parenchyma cells
Guard cells
Trichomes
Periderm
Cork cells
Cork cambium cells
Cork parenchyma
Ground tissue performs many functions:
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Photosynthesis, storage, secretion, flexible and rigid structural support
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Parenchyma cells have thin primary walls; polyhedral cells; function in photosynthesis,
storage and secretion; remain alive at maturity.
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Collenchyma cells have an unevenly thickened primary wall; provide flexible structural
support in soft non-woody organs; cells elongated; remain alive at maturity.
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Sclerenchyma cells have primary and thick secondary walls; provide rigid support to organs;
cells usually die at maturity; fibers are long and tapered; sclereids are short and cubical.
Vascular tissue system conducts materials throughout the plant body and provides support.
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Xylem conducts water and minerals from the roots to all parts of the plant;
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tracheids and vessel elements are the conducting cells and are dead at maturity; both have
wall pits for lateral transport.
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Phloem transports sugars in solution to all plant parts;
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sieve tube members are the conducting cells; companion cells regulate the metabolism of the
sieve tube members;
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both remain alive at maturity; sieve tube members lack nucleus and other organelles.
Dermal tissue system is the outer protective covering of herbaceous plants and the young tender parts of
woody plants.
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Epidermis usually consists of a single layer of parenchyma cells with guard cells and
trichomes; secretes the waxy cuticle; gas exchange occurs through the stomata. It is made of
parenchyma type cells.
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Periderm forms the outer bark of woody plants; cork cells are dead at maturity and filled
with suberin, a water proof substance; cork parenchyma functions as a storage tissue.
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 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.