The Tissue System

The various types of tissues present in the body of a plant perform different functions. Several tissues may collectively perform the same function. A collection of tissues performing the same general function is known as a “Tissue System''. According to Sachs (1975) there are three major tissue systems in plants as follows:

(1) Epidermal tissue system    (2) Ground or fundamental tissue system   (3) Vascular tissue system

(1) Epidermal tissue system: The tissues of this system originate from the outermost layer of apical meristem. It forms the outermost covering of various plant organs which remains in direct contact with the environment.

(i) Epidermis: Epidermis is composed of single layer cells. These cells vary in their shape and size and form a continuous layer interrupted by stomata. In some cases epidermis may be multilayered e.g. Ficus, Nerium, Peperomia, Begonia etc.

The epidermal cells are living, parenchymatous, and compactly arranged without intercellular spaces.

Certain epidermal cells of some plants or plant parts are differentiated into variety of cell types :

(a) In aerial roots, the multiple epidermal cells are modified to velamen, which absorb water from the atmosphere (e.g., Orchids).

(b) Some of the cells in the leaves of grasses are comparatively very large, called bulliform or motor cells. It is hygroscopic in nature. e.g., Ammophila. They are thin-walled and contain big central vacuoles filled with water. They play an important role in the folding and unfolding of leaves.

(c) Some members of Gramineae and Cyperaceae possess two types of epidermal cells : the long cells and the short cells. The short cells may be cork cells or silica cells.

(ii) Cuticle and Wax : In aerial parts, epidermis is covered by cuticle. The epidermal cells secrete a waxy substance called cutin, which forms a layer of variable thickness (the cuticle) within and on the outer surface of its all walls. it helps in reducing the loss of water by evaporation. Usually the cuticle is covered with wax which may be deposited in the form of granules, rods, crusts or viscous semiliquid masses. Other substances deposited on the cuticle surface may be oil, resin, silicon and salts (cystoliths are crystals of calcium carbonate, e.g., Ficus. Druse and Raphides, e.g., Pistia) are crystals of calcium oxalate. Thick cuticle are found in leaves of dry habitats plants.

(iii) Stomata : Stomata are minute apertures in the epidermis. Each aperture is bounded by two kidney shaped cells, called guard cells. Stomata are absent in roots. In xerophytes the stomata are sunken in grooves due to which rate of transpiration is greatly reduced (e.g. Nerium). Usually there is a large air cavity below each aperture, it is called substomatal cavity. In some species the guard cells are surrounded by subsidiary cells or accessory cells which differ morphologically from the other epidermal cells. In monocots e.g., Doob, Maize guard cells are dumb bell shape. Stomata are scattered in dicots leaves but they are arranged in rows in monocots.

Depending upon distribution of stomata, the leaves are :

(a) Apple-mulberry type : e.g. Oxalis, Mulberry, Apple.

(b) Potato type : e.g. Bifacial (dorsiventral leaves of pea, bean, tomato).

(c) Oat type : e.g. Suberect (isobillateral) leaves of most grasses and cereals (monocotyledens).

(d) Nymphea type : e.g. Floating leaves of Nelumbo, Nymphia, water lily.

(e) Potamogeton type : e.g. Submerged plants like Hydrilla, Vallisneria, Potamogeton.

(iv) Trichomes : These are epidermal outgrowths present temporarily or permanently on almost all plant parts. They may be unicellular or multicellular and vary in size and shape in different species. They may be of different types : stellate hair, glandular hair, short glandular hair, floccose hair, urticating hair and stinging hair.

   The trichomes serve for checking excess loss of water and for protection.

(v) Root hairs : They are enlargements of special epiblema cells called trichoblasts and occurs in a particular zone of young root called root hair zone. A root hair cell has vacuolated protoplast with nucleus present towards the apical part of hair. They are specialised to absorb water from soil crevices. They also hold soil particles.

(2) Ground or Fundamental tissue system : Ground tissue system includes all the tissues of plant body except epidermal tissue system and vascular tissues. It forms the bulk of body. This tissue system mainly originates from ground meristem. The ground tissues constitute the following parts :

(i) Cortex : It lies between epidermis and the pericycle. The cortex is distinct in dicotyledons but not in monocotyledons where there is no clear demarcation between cortex and pith. It is further differentiated into :

(a) Hypodermis : It is collenchymatous in dicot stem and sclerenchymatous in monocot stem. It provides strength.

(b) General cortex : It consists of parenchymatous cells. Its main function is storage of food.

(c) Endodermis (Starch sheath) : It is mostly single layered and is made up of parenchymatous barrel shaped compactly arranged cells. The inner and radial or transverse wall of endodermal cells have casparian strips of suberin. In roots thick walled endodermal cells are interrupted by thin walled cells just outside the protoxylem pathches. These thin walled endodermal cells are called passage cells or transfusion cells. A fully developed endodermis is found in all types of roots. Endodermis with characteristic casparian bands is absent in woody dicot stem, monocot stem and leaves of angiosperms. The young stems of angiosperms show a layer with abundant starch deposition. This layer occurs in the position where endodermis would have been situated which is called as starch sheath. In Selaginella trabeculate endodermis is found due to formation of air spaces between two endodermal cells.

Endodermis behave as water tight dam to check the loss of water and air dam to check the entry of air in xylem elements. Endodermis is internal protective tissue.

(ii) Pericycle : It is a single layered or multilayered cylinder of thin-walled or thick-walled cells present between the endodermis and vascular tissues. In some cases, the pericycle is made up of many layers of sclerenchymatous cells (Cucurbita stem) or in the form of alternating bands of thin-walled and thick-walled cells (Sunflower stem). In case of roots, the pericycle is made up of thin-walled parenchymatous cells which later on gives rise to lateral roots. In dicot roots the cork cambium originates in the pericycle which results in the formation of periderm. Pericycle also gives rise to a part of vascular cambium in dicot roots.

(iii) Pith or Medulla : It occupies the central part in dicot stem, and monocot root. It is mostly made up of parenchymatous cells. in dicot root pith is completely obliterated by the metaxylem elements. In dicot stem the pith cells between the vascular bundles become radially elongated and known as primary medullary rays or pith rays. They help in lateral translocation.

(3) Vascular tissue system : The central cylinder of the shoot or root surrounded by cortex is called stele. The varying number of vascular bundles formed inside the stele constitute vascular tissue system. Xylem, phloem and cambium are the major parts of the vascular bundle. Vascular bundle may be of following types :

(i) Radial : The xylem and phloem strands alternate with each other separated by parenchymatous cells. such kinds of vascular bundles are called radial and found mainly in roots.

(ii) Conjoint : A vascular bundle having both xylem and phloem together, is called conjoint. Normally the xylem and phloem occur in the same radius. They occur in stems. Such vascular bundles are of two types :

(a) Collateral : A vascular bundle in which the phloem lies towards outerside and xylem towards inner side, is called collateral, e.g., Sunflower.

Collateral bundle having a cambium between xylem and phloem is said to be of the open type, e.g., Dicot stem.

Collateral bundle lacking a cambium between xylem and phloem is said to be of the closed type, e.g., Monocot stem.

(b) Bicollateral : A vascular bundle having the phloem strands on both outer and inner side of xylem, is called bicollateral. e.g., Cucurbita.

(iii) Concentric : A vascular bundle in which one tissue is completely surrounded by the other, is called concentric. The concentric bundles are of two types :

(a) Amphivasal (Leptocentric) : The phloem lies in the centre and remains completely surrounded by xylem. e.g., Dracaena, Yucca.

(b) Amphicribal (Hadrocentric) : The xylem lies in the centre and remains completely surrounded by phloem. e.g., Ferns.

Internal structure of root, stem and leaf

(1) Functions of different organs and tissues of a plant tissue system