The Tissue System

THE TISSUE SYSTEM

In response to division of labour, tissues are classified into three systems:

A. Epidermal tissue system

It consists of epidermis and its associated structures.

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

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.

Cuticle is absent in roots.

Stomata are structures present in the epidermis of leaves.

Stomata regulate the process of transpiration and gaseous exchange.

Each stoma is composed of two bean shaped cells known as guard cells.

In grasses, the guard cells are dumb-bell shaped.

The outer walls of guard cells (away from the stomatal pore) are thin and the inner walls (towards the stomatal pore) are highly thickened.

The guard cells possess chloroplasts and regulate the opening and closing of stomata.

Sometimes, a few epidermal cells, in the vicinity of the guard cells become specialised in their shape and size and are known as subsidiary cells.

The stomatal aperture, guard cells and the surrounding subsidiary cells are together called stomatal apparatus.

Mostly epidermis is single layered parenchymatous, but is multilayered in leaf of Ficus and Nerium.

Epidermis is mainly protective in nature (external protective tissue).

In grass leaves, motor or bulliform cells are present in upper epidermis.

On stem, the epidermal hairs are called trichomes, which are usually multicellular.

They may be branched or unbranched and soft or stiff. They may even be secretory. These help in preventing water loss due to transpiration.

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In grasses and Equisetum, silica is present in the epidermal cells.

The epidermal cells containing cystoliths are called lithocysts, these are found in Ficus leaves.

B. Ground or fundamental tissue system

It extends from epidermis upto the centre of axis (excluding vascular tissue).

The ground tissue constitutes the following parts :

(a) Cortex. It lies between epidermis and the pericycle. It is further differentiated into

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

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

(iii) Endodermis (called starch sheath in dicot stem). It is mostly single layered and is made up of parenchymatous, barrel shaped, compactly arranged cells. The inner and radial wall of root endodermis cells have casparian strips. These thick walled endodermal cells are interrupted by thin walled cells just outside the protoxylem patches. These thin walled endodermal cells are called passage cells.

Endodermis behaves as water and air tight dam to check the loss of water and entry of air in xylem elements.

(b) Pericycle. It lies between endodermis and vascular tissue. It is mostly single layered and parenchymatous in roots and sclerenchymatous (mixed with parenchyma) in stem. The pericycle cells just opposite the protoxylem are the seat for the origin of lateral roots. In dicot roots, pericycle form a part of cambium and whole of cork cambium.

(c) Pith. 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 crushed by the metaxylem elements. In dicot stem the pith cells between the vascular bundles become radially elongated and are known as primary medullary rays or pith rays. They help in lateral translocation.

C. Vascular tissue system

Vascular bundles found in stelar part constitute vascular tissue system.

Xylem, phloem and cambium forms the major part of the vascular bundle.

Vascular bundles may be of following types -

(a) Radial. When the xylem and phloem are arranged on different radii, alternating with each other, e.g., roots.

(b) Conjoint. When xylem and phloem combine in the same bundles and are present on the same radius, e.g., stem. Conjoint vascular bundles may be:

(i) Collateral. Xylem is towards inner side and phloem towards outside.

(ii) Bicollateral. When xylem is surrounded on its both sides by the phloem and cambium e.g., members of Cucurbitaceae and Solanaceae.

• Open. Cambium is present between xylem and phloem, e.g., dicot stem.

• Closed. Cambium is absent between xylem and phloem, e.g., monocot stem.

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Concentric. When one vascular tissue surrounds the other. They are of two types:

(i) Amphicribal or Hadrocentric. The xylem is surrounded on all sides by phloem e.g., ferns.

(ii) Amphivasal or Leptocentric. The phloem is surrounded on all sides by xylem e.g., Yucca, Dracaena.