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TISSUES
Chapter 4
Plants have three or four major sets of organs: roots, stems, leaves and, in some instances,
flowers.
Three basic tissue patterns occur in plants: woody dicots, herbaceous dicots and
monocots.
The principal tissues of plants are grouped together into larger units called tissue systems
based on their continuity throughout the plant body.
1. Dermal tissue system
2. Vascular tissue system
3. Ground tissue system
Each organ, - stem, leaf and root - has three tissue systems.
Tissues are group of cells that are structurally and/or functionally distinct.
Plant tissues composed of only one type of cell are called simple tissues, and those made
of two or more types of cell are called complex tissues.
Each tissue system is continuous throughout the plant body, but their arrangement varies in
the different organs.
Plant tissues originate in the meristematic regions (meristems) of the plant where active cell
division takes place.
MERISTEMATIC TISSUES
Growth in plants is localized in regions called meristems.
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Plants have indeterminate growth; they grow for as long as they live.
Plants have perpetually embryonic cells capable of cell division.
Growth involves cell division, cell elongation and cell differentiation.
Some cells remain as part of the meristem and continue to divide.
APICAL MERISTEMS
Apical meristems are found at the tip of roots and stems and are responsible for the
extension of the plant body.
Initials are cells that divide and produce one body cell, the derivative, and another cell that
remain in the meristem.
Derivative cells divide near the root tip and produce three primary tissues that remain
meristematic for some time before becoming differentiated.
These meristematic tissues are the protoderm, ground meristem and procambium.
The protoderm, ground meristem and procambium are partly differentiated tissues capable
of cell division.
Activity of the initials and the three primary meristems constitutes the primary growth of
the plant that will produce the primary plant body.
Plants continue to grow throughout their entire lifetime. They have indeterminate growth.
LATERAL MERISTEMS
The vascular cambium and cork cambium are lateral meristems.
Lateral meristems increase the girth of roots and stems.
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 and forms a cylinder that extends into the
roots and branches of the plant.
The cambium cylinder is made of initials; the derivatives differentiate into several types of
cells.
Secondary tissues are called so because they are produced after the primary tissue has
matured.
Grasses and its relatives lack vascular and cork cambium.
They have apical meristems, and, in the vicinity of the nodes, they have what is called
intercalary meristems.
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Nodes are the areas where the leaves attach to the stem.
TISSUES PRODUCED BY MERISTEMS
Some tissues are made of one type of cells, simple tissues; others are made of several
types of cells, complex tissues.
SIMPLE TISSUES
Parenchyma tissue
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Made of parenchyma cells
Living cells at maturity.
Have thin primary walls.
Function in storage, secretion and photosynthesis.
Usually have large vacuoles, and may have tannins, crystals, oils, starch grains, etc.
Found throughout the body of the plant.
This is the most abundant tissue in plants.
Aerenchyma is a modified parenchyma with large intercellular spaces forming a
network.
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Chlorenchyma is name given to parenchyma cells containing many chloroplasts.
Transfer cells are parenchyma cells with wall ingrowths that are involved in the
movement of large amounts of solutes over short distances. They are found in the
xylem and phloem of small veins, the placenta, endosperm and other reproductive
structures, in glandular tissues (nectaries, salt glands)
When the tissue is damage, parenchyma cells may start dividing again.
Collenchyma cells
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Living cells at maturity.
Have unevenly thickened primary cell walls in the corners.
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.
Secondary walls impregnated with lignin.
Secondary wall with pits.
Cells are often dead at maturity.
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.
COMPLEX TISSUES
Complex tissues are produced by the meristems (primary complex tissues) and by the
cambium layers (secondary complex tissues).
Apical meristems produce the following tissues:
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Epidermis (dermal tissue system).
Ground tissues made of parenchyma, sclerenchyma and collenchyma (ground
tissues system).
Primary xylem and primary phloem (vascular tissue systems)
1. 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
XYLEM
Function:
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Xylem conducts water and minerals from the roots to all parts of the plant; it also
supports the plant and stores food and other organic substances.
Structure:
There are two tracheary elements that make the xylem: tracheids and vessel elements.
Tracheids and vessel elements are the conducting cells and are dead at maturity; both
have wall pits on their sidewalls for lateral transport.
Xylem vessels arise from individual cylindrical cells oriented end to end. At maturity the
cytoplasmic contents die.
The secondary walls of the xylem vessels are deposited in spirals and rings and are
usually perforated by pits.
At maturity, vessel elements have perforations, which are areas lacking primary and
secondary walls. These perforations or holes occur on the end walls of the vessel elements.
The result is the xylem vessel, a continuous nonliving duct.
Xylem also contains tracheids. These are individual cells tapered at each end so the
tapered end of one cell overlaps that of the adjacent cell.
Like xylem vessels, tracheids have thick, lignified walls and, at maturity, no cytoplasm.
Their walls are perforated so that water can flow from one tracheid to the next. There are
thin membranes in the pit that prevent air bubbles from passing to the adjacent tracheid.
The xylem of ferns and conifers contains only tracheid
Xylem text: http://www.biologie.uni-hamburg.de/b-online/e06/06b.htm
Xylem elements: http://www.biologie.uni-hamburg.de/b-online/e06/mazerirt.htm
Xylem pits: http://www.biologie.uni-hamburg.de/b-online/e06/hoftupf.htm
PHLOEM
Function:
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Phloem transports sugars in solution to all plant parts.
Structure:
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sieve tube members found in angiosperms; have sieve plate and companion cells.
Sieve tube elements are so-named because their end walls are perforated forming the
sieve areas or sieve plates. This allows cytoplasmic connections between vertically stacked
cells. The result is a sieve tube that conducts the products of photosynthesis - sugars and
amino acids -from the place where they are manufactured (a "source"), e.g., leaves, to the
places ("sinks") where they are consumed or stored.
Sieve tube members have only primary wall.
Sieve members remain alive at maturity; sieve tube members lack nucleus, vacuoles, Golgi
complex, ribosomes and cytoskeleton.
Plasma membrane and endoplasmic reticulum remain. ER is particularly abundant near the
sieve plates.
Callose, a polysaccharide of glucose, is deposited in the pores of injured sieve elements.
Sieve tube members are the conducting cells; companion cells regulate the metabolism of
the sieve tube members. Both are derived from the same mother cell in angiosperms.
Companion cells move sugars and amino acids into and out of the sieve elements.
There are numerous cytoplasmic connections (plasmodesmata) between the companion
cells and the sieve tube members.
Companion cells are found only in angiosperms.
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sieve cells found in seedless plants and gymnosperms; have narrow pores but lack
plates; accompanied by albuminous cells of different origin from companion cells.
Albuminous cells are parenchyma cells found in the phloem of gymnosperms and
pteridophytes. They are not derived from the same mother cell that gives rise to sieve tube
members.
It is thought that albumimous cells perform the same function as companion cells.
Parenchyma cells are also found in the phloem and are associated with the storage of a
variety of substances.
Fibers and sclereids are also present in the phloem and help in supporting the plant body.
Phloem text: http://www.biologie.uni-hamburg.de/b-online/e06/06d.htm#04
Sieve tubes and companion cells: http://www.biologie.uni-hamburg.de/b-online/e07/16.htm
2. 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
Dermal tissue system is the outer protective covering of herbaceous plants and the young
tender parts of woody plants, the primary plant body.
The epidermis covers leaves, floral parts, fruits, seeds, stems and roots until they have
undergone considerable secondary growth.
EPIDERMIS
Functions:
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Protection against various chemical and physical influences, against being fed upon
by animals and against infestation by parasites, against desiccation
Involved in gas exchange,
Secretion of metabolic compounds
Absorption of water
Site of receptors for light and mechanical stimuli that help to transform signals from
the surrounding to the plant
Structure:
The epidermis has differentiated cells to perform the functions mentioned above:
parenchyma epidermal cells, parenchyma storing cells, guard cells and trichomes.
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.
Plants that produce aerial roots may have an epidermis several cells thick called velamen,
e.g. orchids.
The outer layer of the velamen functions as a sponge.
Some tropical plants have a multiple-layered epidermis in their leaves to prevent
desiccation, e. g. tropical figs (Moraceae), and some members of Piperaceae (Pepper
family).
Plant cuticle is composed of a structural polymer, cutin that is embedded in a complex
mixture of highly hydrophobic soluble materials called waxes.
Cuticular waxes are complex substances made of lipids and esters and that vary from
species to species.
Wax may form a smooth sheet, folds or rod-like deposits on the surface of the epidermis.
“These cuticle folds have several functions: they enhance the intense velvet effect of the flower color, increase
the water-repellent quality of the flower surface, strengthen the stability of the petals and finally does it seem as
if they could be recognized by landing pollinators. It supplies the insects with an additional information
concerning the 'right' landing strip. (W. BARTHLOTT, N. EHLER, 1977, W. BARTHLOTT; 1981)
Cuticular waxes folds:
http://www.biologie.uni-hamburg.de/b-online/e05/r03.htm
http://www.biologie.uni-hamburg.de/b-online/e05/r04.htm
Root hairs are involved in water and mineral absorption.
Trichomes have a variety of functions: secretion of protective chemicals, provide a barrier
to insect attack, secretion of salts in some species, absorption of water in epiphytes, etc.
Some epidermal cells may be modified as glands that secrete protective chemicals.
“The stinging hairs of the stinging nettle are really multi-cellular trichomes. They consist of two parts and a
multi-celled base that develops not only from epidermal cells, but from those of the subepidermal layers, too. A
hair cell is sunken into the base. Its basal part is called bulbus. It is surrounded by the cells of the base like
liquid is surrounded by a mug. Its upper part is elongated and thin and ends in a laterally attached head. At the
site of transition, the cell wall is considerably thinner than in other parts of the cell. Encrusting silicates make it
brittle and cause the head to break off easily when touched. This leaves the hair with a point that bears an
astonishing resemblance to the needle of a syringe. The pressure of the touch is directly transferred to the
bulbus due to the rigidity of the cell wall. It presses the content of the bulbus (sodium formamide, acetyl choline,
histamine) through the cannula and injects it into the wound.”
Source: http://www.biologie.uni-hamburg.de/b-online/e05/05a.htm#14
Epidermis text: http://www.biologie.uni-hamburg.de/b-online/e05/05a.htm
Stomata: http://www.biologie.uni-hamburg.de/b-online/e05/r07.htm
PERIDERM
Function:
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Periderm replaces the epidermis in stems and roots having secondary growth. It
forms the outer bark of woody plants.
Structure:
The periderm consists of three layers:
1. phellogen (cork cambium) produces the secondary growth.
2. phellem (cork cells) produced by the phellogen towards the outside and forms the
outer layer of the tree bark.
3. phelloderm (living cork parenchyma) produced by the phellogen inwardly and forms
part of the inner bark of the tree. The phelloderm is not always produced.
The periderm consists mostly of cork or phellem. Cork cells are dead at maturity and filled
with suberin, a waterproof substance.
Loosely arrange surface peridermal cells form lenticels that contribute to aeration of the
stem. These cells lack suberin.
The periderm also includes cork cambium or phellogen and the phelloderm or living
parenchyma.
The cork cambium forms cork on its outer surface and phelloderm on its inner surface.
Cork parenchyma or phelloderm functions as a storage tissue.
Bark refers to all tissues external to the vascular cambium: secondary phloem, cork
cambium and cork (phloem plus periderm).
Periderm text: http://www.biologie.uni-hamburg.de/b-online/e05/05c.htm#01
SECRETORY CELLS AND TISSUES
Secretory cells may function individually or as part of a secretory tissue.
Secretory tissues are often derived from parenchyma cells and are located in different
places in the plant.
These tissues are involved in secondary plant metabolism.
The term basic metabolism comprises all pathways necessary for the survival of the cells,
while secondary plant products are such that occur usually only in special, differentiated
cells and are not necessary for the cells themselves but may be useful for the plant as a
whole.
These secondary products include latex, alkaloids, tannins and many other compounds
often involved in defending the plant from herbivores.
Secondary metabolites: http://www.biologie.uni-hamburg.de/b-online/e20/20.htm#09
Helpful pictures with explanation:
http://www.botany.org/plantimages/Angiosperms-Magnoliophyta.php
http://www.sbs.utexas.edu/mauseth/weblab/table_of_contents.htm