Short essay on the tissue system

In higher plants tissues are arranged into different tissue systems. Sachs, a German Scientist for the first time in 1875 classified the plant tissue system into three basic categories basing on their position and morphology.

These are: (1) epidermal tissue system, (II) the ground/fundamental tissue system, and (III) the vascular tissue system. Each system may consist of only one or a combination of tissues that may be structurally alike or of different nature, but perform a common function and share a common origin.

l. The Epidermal Tissue System:

The epidermal tissue system consists mainly of epidermis, stomata, the epidermal appendages, root hairs etc.

The epidermis (epi, upon; derma, skin) is the outermost covering of the primary plant body. It originates from the protoderm. (In shoot, apices with distinct tunica and corpus layers, epidermis develops from the outer layer of tunica).

The epidermis is usually single layered but multi layered epidermis also exists as in the leaves of Ficus, Begonia, Nerium etc. In old stems, after secondary growth epidermis is replaced by periderm.

The epidermis is made up of elongated compactly arranged cells forming a continuous layer without intercellular spaces except in petals of plants like Linum. The cells appear more or less rectangular in transection.

The epidermal cells Are parenchymatous in nature with a large central vacuole surrounded by a thin layer of colorless cytoplasm.

The outer or exposed walls of epidermal cell are thick and usually covered by a cuticle formed by the deposition of a waxy material secreted by epidermal cells.

The outermost layer in root is referred to as epiblema or piliferous layer due to its structure and nature of origin. Epiblema is neither cutinized nor provided with stomata.

The epidermises in some monocots like the members of the family Poaceae contain builliform or motor cells. These are thin walled, larger cells helping in rolling and unrolling of the leaves by changes in their sizes, depending on the inflow and outflow of water.

Several minute openings called stomata (sing, stoma) are found in the epidermis of young, green, aerial parts and leaves.

Each opening is surrounded by a pair of guard cells. Guard cells, unlike epidermal cells contain chloroplasts and are responsible for the opening and closing of stoma. Sometimes, a few neighboring epidermal cells, in the vicinity of guard cells become specialized in their shape, size and contents and are known as I subsidiary cells.

In such case, the stomatal aperture together with the guard cells and the subsidiary cells are called stomatal apparatus. On the basis of the number and arrangement of subsidiary cells the stomata can be of following six types.

(i) Anomocytic- Indistinguishable subsidiary cells Ex; Malvaceae, Ranunculaceae, Papaveraceae.

(ii) Anisocytic – Three subsidiary cells surround, the stoma. Ex-Cruciferae, Solanum, Nicotiana.

(iii) Diacytic – Two .subsidiary cells surround the stoma with there common wall at right angles to guard cells.

(iv) Paracytic – One or more subsidiary cells (usually two) with their longitudinal axis parallel to guard cells. Ex-Rubiaceae.

(v) Antinocytic – Four or more radially elongated subsidiary cells, Ex.-Musaceae Commelinaceae

(vi) Cyclocytic – Four or more subsidiary cells arranged in a narrow ring around stoma. Ex,-Palmaceae.

When the subsidiary cells and guard cells develop from the same mother cells then the stomata are mesogenous (Hyplocheilic). If the subsidiary and guard cells have independent origin the stomata are perigenous (syndetocheilic).

In mesoperigenous condition, at least one subsidiary cell has a common origin with the guard cells and the rest do not. The distribution of stomata in leaves vary greatly. In most of the dicot leaves the stomata are confined to the lower surface of the leaf. Such leaves are called hypostomatic.

In some plants like Polygonum amphibian, having floating leaves stomata are confined to upper surface of leaf. Such leaves are called epistomatic.

In monocots stomata are found on both surfaces such leaves are amphistomatic. In plants growing in deserts or dry regions, stomata occur sunken in pits to reduce excessive transpiration.

The cells of the epidermis give rise to a number of appendages which vary greatly in their shape, structure and function. These are known as trichomes. These appendages may be multi cellular or unicellular.

The unicellular trichomes are usually simple, unbranehed, elongated structures. The multi cellular trichomcs and glands are made up of several layers of cells. There are four main types of trichomcs: (i) Hairs, (ii) collectors, (iii) water vesicles or bladders and (iv) scales.

The multi cellular hairs are associated with stem epidermis while unicellular hairs are found on roots. I Hairs can also be branched or unbranehed. Collectors are glandular hairs with multi cellular stalk and head.

They are found in bud scales and stipule. Water vesicles, otherwise known as vesicular hairs are greatly distended epidermal cells storing water. Scales are modified trichomes with discoid sheet or plate of cells mounted on stalk. Besides trichomes epidermal emergences like spines, warts are also found in plants.

Epidermal root hairs are extension of individual epidermal cells, which occur in a zone just behind the tips of young growing roots. Because a root hair is simply an extension and not a separate cell, there is no cross-wall isolating it from the epidermal cell.

The root hair is formed due to elongation of epidermal cell, and is therefore, not a protuberance or appendage. The root hairs are ephemeral (short-lived) structures which play an important role in anchoring the plant body in the soil and absorbing water and minerals from the soil.

I. Function:

(i) Protection of inner tissue.

(ii) Cuticularisation of epidermis, checks transpiration. Presence of cuticle also reduces heating by reflecting light to certain extent.

(iii) Stomata are responsible for exchange of gases.

(iv) Bullform cells help in rolling and unrolling of leaves thus control transpiration.

(v) Different appendages carry out specialized functions like scale for protection, water vesicles for retention of water etc. glands for secretion etc.

(vi) Root hairs are responsible for absorption.

(vii) Epidermal hairs on seeds help in dispersal.

II. The Ground Tissue System (Fundamental Tissue System) :

The ground tissue system or fundamental tissue system forms the main bulk of the plant body. It extends from below the epidermis up to the central region. This system has various kinds of tissues such as parenchyma, collenchymas and sclerenchyma of which parenchyma is the most abundant. It is differentiated into following zones and sub-zones.