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The following points highlight the three main types of tissue system in plants. The types are: 1. Epidermal Tissue System 2. Ground Tissue System 3. Vascular Tissue System.
Type # 1. Epidermal Tissue System:
This tissue system forms the outermost covering of plant body. It is derived from protoderm. It consists of epidermis and epidermal appendages. Epidermis is made of epidermal cells and stomata.
a. Epidermis (Gk. epi— upon, derma— skin):
Epidermis is the outermost protective layer of primary plant body. It is usually single layered. Multi-layered epidermis occurs in the leaves of some tropical plants (e.g., Oleander, Banyan) and aerial roots of orchids. Epidermis is a conspicuous layer of elongated, compactly arranged living cells which do not enclose intercellular spaces. The cells possess large central vacuoles and thin peripheral cytoplasm.
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They remain thin walled in roots and plants growing under moist conditions. The root epidermis is also called piliferous layer because it bears root hairs. Epidermal cells of the aerial parts of the plants have wavy lateral walls in dicots and straight walls in monocots.
Their outer walls are cutinised. Cutin is a fatty-waxy substance. The cutinised walls are less permeable to water. The impermeability depends upon the thickness of cutin. Cutin also forms a separate layer on the outside of epidermis. It is called cuticle.
Under extremely dry conditions the cuticle is reinforced by a layer of wах. Wax produces mealy coating or bloom. Wax is also present on the upper surface of floating leaves. It protects the floating leaves from wetting. In cereals the epidermal cells have a deposition of silica. Silica provides stiffness. It is also abrasive and hence protective against grazing.
The epidermis of aerial parts usually bears a number of minute pores called stomata. Stomata are absent in the surface layer of roots. They are fewer in case of stems but are abundant in case of leaves. Each stoma (singular of stomata) or stomate is surrounded by a pair of specialised epidermal cells called guard cells. Guard cells differ from rest of the cells in shape, size and thickenings.
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They also have a few small chloroplasts. The guard cells are generally bean or kidney shaped in most plants. They are dumb-bell shaped in grasses. Inner walls of the guard cells (towards the stamatal pore) are thick while the outer ones are thin.
Stomata regulate transpiration and gaseous exchange with the help of their guard cells. The latter expand and contract in response to their turgidity and thus open or close the stomatal aperture.
In some cases the guard cells are surrounded or overtopped by another category of less modified epidermal cells called subsidiary cells. When subsidiary cells lie above the guard cells, the stomata are called sunken. Stomatal aperture, guard cells and subsidiary cells together constitute a complex called stomatal apparatus.
In dorsiventral leaves the stomata occur mostly on the lower surface (hypostomatic) but in vertically oriented leaves and the plants growing in moist environment, they occur on both upper and lower sides (amphistomatic). In floating leaves of aquatic plants, they are restricted to the upper surface only (epistomatic).
b. Epidermal Appendages:
They are of two types, trichomes and emergences:
Trichomes:
They are unicellular or multicellular outgrowths which are strictly epidermal in origin. Trichomes are of two kinds, hair and scales. Hairs are elongated structures and can be unicellular or multicellular. Scales are multicellular flattened structures (e.g., Nepenthes).
(i) Root Hairs (Fig. 6.16A):
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They are unicellular tubular structures found in epiblema of root in a special area called root hair zone. Root hairs are actually not appendages or protuberances but are simply enlargements of epiblema cells. Root hair cells have vacuolated protoplasm.
Nucleus occurs towards the apical part of the hair. Wall is thin and pectocellulosic. Root hairs are ephemeral. New root hairs are continuously developed on young parts of the root. Root hairs take part in absorption of water and mineral salts. They also hold the soil particles and play an important role in anchoring the plant.
(ii) Aerial Hairs (Fig. 6.16 B-E):
They are unicellular or multicellular appendages which are covered by a layer of cuticle. Unicellular hairs are simple and often un-branched. Multicellular hair are more abundant. They can be un-branched or branched. The hair enclose stationary air and protect the plant organs against sudden changes of temperature and high rate of transpiration.
Cotton is obtained from long unicellular epidermal hair or lint (c.f., fuzz) of Gossypium seeds. A single seed may have up to 1000 lint hair (Fig. 6.17). One kg of cotton contains over 200 million hair. The hair have cellulose thickening.
(iii) Stinging Hairs:
They are hollow hairs that contain siliceous tips and enclose a poison which is injected into the skin of animals rubbing against them, e.g., Urticadioica (Fig. 6.18 B, Stinging Nettle).
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(iv) Glandular Hairs:
Most of the glandular trichomes produce essential oils (Fig. 6.18 A). They provide characteristic odour to plants, e.g., Citrus, Mint. The digestive glands of insectivorous plants are also trichome in nature.
Emergences:
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They are multicellular epidermal outgrowths which also contain some inner tissues. Prickles are an example of emergences. They are sharp and stiff outgrowths. Prickles do not have vascular supply. They protect the plant from excessive transpiration, grazing animals and in some roses help the plants in climbing.
Functions:
(i) Being the outermost layer, it is protective in nature,
(ii) It forms water and mineral absorptive system of the root,
(iii) With the help of cuticle it checks the rate of water loss from aerial parts,
(iv) Presence of epidermal hair form an insulating layer over the surface,
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(v) Prickles and stinging hair protect the plant from herbivores,
(vi) Glandular hairs provide aroma to the plants,
(vii) Stomata take part in exchange of gases and transpiration.
(viii) Trichomes present on the surface of some seeds and fruits help in their dispersal.
Type # 2. Ground Tissue System:
The system is formed from ground meristem or partly plerome and partly periblem that forms the interior of plant organs with the exclusion of epidermal and vascular systems.
It consists of simple permanent tissues like parenchyma, collenchyma and sclerenchyma. Ground tissue system of leaves is called mesophyll. Mesophyll is made up of two types of photosynthetic cells, palisade and spongy. Palisade parenchyma occurs towards the upper surface.
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It is formed of columnar cells. Abundant chloroplasts occur in these cells. Intercellular spaces are quite narrow. Spongy parenchyma occurs towards the lower epidermis and encloses large Intercellular spaces. Its cells are rounded, isodiametric, angular or lobed. They contain good number of chloroplasts.
The ground system of monocot stem has two parts, hypodermis and ground parenchyma. In roots and diet stems, the ground tissue system is differentiated into hypodermis, cortex, endodermis, pith and medullary rays. Pericycle is actually constituent of vascular tissue system but is often included in ground tissue system.
a. Hypodermis:
It forms a few layers of collenchyma or sclerenchyma that lies below the epidermis. It provides mechanical strength and rigidity. In aerial stems it additionally functions as heat screen.
b. Cortex:
Cortex is commonly thin-walled parenchymatous region that lies between endodermis and hypodermis/epidermis. It stores food and performs some additional functions like enclosing large air cavities in aerenchyma and performing photosynthesis if chlorenchymatous.
In stem the primary function of cortex is the formation of protective zone. Its secondary function is storage of food. Accessory functions include photosynthesis and retention of gases if aerenchymatous. In roots, the primary function of cortex is storage of food. In root hair zone it is transfer of absorbed water and minerals to the interior.
c. Endodermis:
Endodermis is the innermost layer of cortex that consists of tightly packed barrel shaped cells. It is called starch sheath in case of dicot stems. In roots its cells possess lignosuberin casparian strips or bands. Major function of endodermis is to act as check post between vascular strand and cortex.
d. Pericycle:
It is the outer boundary of vascular strand that is one to several cells in thickness. In roots it gives rise to lateral branches. Part of vascular cambium is also formed by it. Pericycle of young roots is made up of thin walled cells.
In stem pericycle may be parenchymatous, sclerenchymatous or both. Sclerenchymatous pericycle is both protective and supportive. Parenchymatous pericycle helps in exchange of material between cortex and vascular bundles. Pericycle is absent in stems and roots of aquatic plants.
e. Pith:
It lies in the centre and is often parenchymatous. It is well developed in dicot stems and monocot roots. Intercellular spaces may or may not be present. Pith is repository of many excretory substances like tannins, phenols, calcium, etc. It may also store food.
f. Medullary Rays:
They are non vascular areas which occur between vascular bundles in dicot stems for lateral conduction.
Type # 3. Vascular Tissue System:
It forms a strand of vascular tissues that is known as vascular strand or vascular cylinder. In gymnosperms and flowering plants, the vascular tissues occur in distinct patches called vascular bundles. The latter are radial in roots and conjoint in case of stems and leaves (Fig. 6.19).
a. Radial Bundles:
Here xylem and phloem occur in the form of separate bundles called xylem bundles and phloem bundles. The two types of bundles usually alternate with each other. They occur on different radii. Radial bundles are characteristic of roots.
b. Conjoint Bundles:
The vascular bundles which contain both xylem and phloem are called conjoint vascular bundles.
Conjoint bundles are of the following three types:
(i) Collateral Bundles:
They are those conjoint bundles in which phloem and xylem lie together on the same radius with phloem on the outer side and xylem towards the inner side. In gymnosperms and dicot stems a strip of vascular cambium occurs between phloem and xylem of each vascular bundle.
It is called intra-fascicuiar (or fascicular) cambium. This strip of vascular cambium later produces secondary tissues. Such vascular bundles are described as open because the original or primary phloem and xylem separate on the production of secondary tissues by vascular cambium. In monocot stems vascular bundles do not have a strip of vascular cambium. They are termed as closed.
(ii) Bicollateral Bundles:
Bicollateral vascular bundles have phloem both on the outer and inner side of xylem. All the three lie on the same radius. Usually a strip of vascular cambium is present on both outer and inner sides of xylem. Bicollateral bundles occur in Cucurbitaceous (e.g., Pumpkin or Cucurbita pepo, Ridge gourd or Luffa cylindrica) and some members of families Solanaceae, Convolvulaceae, etc.
(iii) Concentric Bundles:
Here out of the two types of vascular tissues (phloem and xylem), one forms a solid core while the other surrounds it completely on all sides. A strip of vascular cambium is always absent.
Concentric bundles are of two kinds:
(a) Amphicribal (Hadrocentric) Bundle. Xylem forms a central core while phloem surrounds it on all sides. It occurs in some aquatic angiosperms and the staminal bundles of many dicots (e.g., Prunus).
(b) Amphivasal (Leptocentric) Bundle. Phloem lies in the centre of the vascular bundle which is completely surrounded by xylem, e.g., Dracaena, Yucca.
