PAB 307: PLANT ANATOMY CLASSIFICATION, DESCRIPTION AND FUNCTION OF PLANT TISSUES General Introduction Cells, the fundamental units of life, are associated in various ways with each other, forming coherent masses, or tissues. Thus, a tissue may be defined as a group of cells of the same or mixed type, having a common origin and performing an identical function (Dutta, 1964). Tissues composed of only one type of cell are called simple tissues, while those composed of two or more types of cells are called complex tissues. In addition, the principal tissues of vascular plants are grouped into larger units on the basis of their continuity throughout the plant body. These larger units, known as tissue systems, are readily recognized, often with unaided eye. Classification of Tissues Tissues may be broadly classified into two groups: meristematic and permanent. A) Meristematic Tissues The meristematic tissues are composed of cells in a state of division and capable of dividing. They may be classified according to their position in the plant body as apical, intercalary and lateral. According to their functions, they may be classified as protoderm, procambium and ground or fundamental meristem. The cells in these tissues are essentially alike and isodiametric. The protoplasm in them is abundant and active with large nuclei, and the vacuoles are small or absent. B) Permanent Tissues These are composed of cells that have lost the power of dividing, having attained their definite form and size. They may be living or dead, thin-walled or thick walled. They are formed by differentiation of cells of the meristem, and may be primary or secondary. The primary permanent tissues are derived from the apical meristems of the stem and root, and secondary permanent tissues from the lateral meristems. Permanent tissues may also be classified into simple and complex tissues, based on cell type composition. Examples of simple permanent tissues include parenchyma, collenchyma and sclerenchyma; while complex permanent tissues include xylem and phloem. The Tissue System On the principle of division of labour, plant tissues are arranged in three systems, each playing a definite role in the life of a plant. Each system may consist of only one tissue or a combination of tissues which may be structurally similar or of different nature, but performs a common function and have the same origin. The three systems are: I. Epidermal tissue system II. Ground or fundamental tissue system III. Vascular tissue system As earlier mentioned, the precursors of these tissue systems are meristems which may be outlined as follows: Protoderm Epidermis Epidermal tissue system Ground meristem Cortex, pericycle, medullary rays, pith Ground tissue Procambium Vascular bundles Vascular tissue system I. Epidermal Tissue System The epidermal tissue system is derived from the dermatogen of the apical meristem and forms the epidermis, or outermost skin layer which extends over the entire surface of the plant body, and basically functions as a protective layer. It is continuous, except for certain openings (stomata and lenticels). The epidermis is mostly single layered, but sometimes may form multiseriate layers, and called multiple epidermis (as found in leaves of Ficus, Indian rubber plant, Piperomia, etc.). When viewed from the surface, they appear somewhat irregular in outline, varying in shape and size, but mostly appear in rectangular form in cross-section. Epidermal cells are mostly composed of parenchyma cells (parenchymatous), compactly arranged without intercellular spaces. They possess relatively small amounts of cytoplasm lining the cell wall and large vacuoles filled with colourless cell sap. The outer cell walls are unevenly thick and impregnated with cutin or suberin, while inner and radial cell walls of the epidermis are mostly thin. The cutinized outer layer (called cuticle) acts as a protective barrier, shielding inner cells against water loss, mechanical injury and potential parasites. Additional waxy substances in form of rods, scales and grains which are deposited on the cuticle help to prevent further loss of water. In many plants, the epidermal cells often bear outgrowths, known as hairs or trichomes. These may be unicellular or multicellular, simple or branched, soft or sharp and stiff. The epidermis may also bear stinging hairs, glandular hairs or dense coating of hair. The epidermis of root without cuticle and not provided with stomata is called epiblema or piliferous layer. The outer walls of most of its cells extends outwards to form tubular, unicellular prolongations called root hairs, which serve to increase absorbing surface of the root organ in the soil. Functions of epidermis Protection of internal tissues against mechanical injury, injurious thermal changes, high wind velocities, attack of microbial parasites, acid rain effects, etc. Reduces loss of water due to excessive transpiration Presence of sharp and stiff hairs on epidermal surface of some plants protects them from grazing Epidermis of some xerophytic plants store food and water materials in the form of mucilage In lower plants, epidermal cells contain chloroplasts and thus manufacture food material Special epidermal structures 1. Lithocytes: Epidermal cells of plants belonging to families like Curcubitaceae and Urticaceae contain deposits of calcium carbonate crystals, arranged in the form of bunches, like those of grapes. Such deposits are called cystoliths and the container cells are called lithocytes. Lithocytes are usually larger than other epidermal cells. 2. Stomata: These are very minute openings formed in the epidermal layer in the aerial parts of plants, mostly the leaves. A pore, the stoma, is surrounded by two specialized, chloroplast containing epidermal cells, called guard cells. The guard cells are kidney shaped in dicotyledonous plants, but dumble-shaped in monocotyledons. They function to guard the passage (stoma) by regulating the opening and closing of the stoma-like lips. Sometimes, the guard cells are surrounded by two or more cells distinct from other epidermal cells, called accessory or subsidiary cells. Stomata (pl. of stoma) occur scattered in dicot leaves, while in monocot leaves, occur in parallel rows. Under normal conditions, they remain closed at night in the absence of light and stay open during the daytime, in the presence of light. They mainly function in the interchange of gases between the plant and the atmosphere – oxygen for respiration and carbon dioxide for manufacture of carbohydrates. 3. Trichomes: These refer to different types of unicellular and multicellular extensive appendages of the epidermis which have a variety of functions. They are important for regulating moisture exchange with the atmosphere, secrete chemicals to provide defense against insect attack and also to repel animals from grazing on the plants. II. Ground or Fundamental Tissue System This system forms the main bulk of the plant’s body and extends from below the epidermis to the centre, excluding the vascular bundles. It is heterogenous in nature, composed of different types of tissues concerned with different functions, of which parenchyma is the most abundant. The other tissues are sclerenchyma and collenchyma, and sometimes lactiferous or glandular tissue. In a stem, it basically includes cortex, pericycle, pith and medullary rays and in a leaf, mesophyll tissues are included in the system. The various sub zones of the system are outlined as follows: i. Cortex: This is the zone that lies between the epidermis and the pericycle, varying in thickness from a few to many layers. Its chief function is to act as a protective tissue in plant stems, while also performing functions of food storage and participation in metabolic activities (photosynthesis). In a typical dicot stem, it is generally differentiated into hypodermis, consisting of a few layers of collenchyma, or sometimes sclerenchyma, and situated just beneath the epidermis. The next layer is the general cortex, made up of few layers of loosely arranged, cortical parenchyma with intercellular spaces. Starch and different types of grain are usually found in these cells, while other cells such as chloroplasts, sclereids, tannin cells, resin ducts, and oil cavities may or may not be present. The innermost layer of the cortex is the endodermis, which is also known as starch sheath, as it often has numerous starch grains. It comprises a single layer of vertically elongated, modified parenchyma cells, which appear in cross-section, as barrel shaped cells without intercellular spaces. The layer is wavy in stems and often not distinguishable, while it is circular and well defined in roots. The cells are living and contain abundant protoplasm, large nuclei, and often starch grains. Some cells of the endodermis may also contain gums, tannins, etc. The endodermis is distinctly present in stems of majority of herbaceous angiosperms, aquatic plants, creepers and rhizomes. It remains absent in woody stems of dicotyledons and gymnosperms, and leaves of angiosperms. ii. Pericycle: This forms one or several cell layers found between the endodermis and vascular bundles. This region surrounds the vascular tissue and is regarded as the limiting region of the stele. It may be homogenous, consisting only of parenchymatous cells, as found in all roots and the pteridophytes. It may also be heteeogenous in nature, consisting of parenchyma and sclerenchyma, the latter forming isolated strands or patches in it. The pericycle occurs uniformly