The plant body consists of various types of cells which form different types of tissues. The structure and function of these tissues depend entirely on where they are located inside a plant. Therefore, based on this, plant tissue systems can be classified into three types – epidermal tissue system, ground or fundamental tissue system, and vascular or conducting tissue system.
Epidermal tissue system:
This tissue forms the outermost covering of the plant body. It consists of epidermal cells, stomata and epidermal appendages including trichomes and hairs. Epidermis refers to the outermost layer of the primary plant body. It is a continuous layer of elongated and compactly arranged cells. Epidermal cells are parenchymatous in nature and their cell walls are lined with a small amount of cytoplasm that has a large vacuole.
The outer layer of the epidermis is coated with a thick and waxy layer called the ‘cuticle’, which prevents the loss of water. Stomata are present in the epidermis of the leaves. They regulate the process of transpiration and gaseous exchange. Each stoma consists of two bean-shaped cells called ‘guard cells’. Guard cells contain chloroplasts and regulate the opening and closing of a stoma. In grasses, the guard cells are dumb-bell shaped. When the epidermal cells associated with the guard cells become specialised in shape and size, they are known as ‘subsidiary cells’. The stomatal aperture, guard cells and the surrounding subsidiary cells are collectively called ‘stomatal apparatus’.
The epidermal tissue has epidermal appendages such as hair and trichomes. The root hairs are unicellular elongations of the epidermal cells and they help absorb water and minerals from the soil.
The epidermal hairs on the stem are known as trichomes. They may be branched, unbranched, soft or stiff and may also be secretory in function. Besides this, trichomes prevent loss of water due to transpiration.
The ground tissue system:
The ground tissue, in primary stems and roots, consists of parenchymatous cells found in the cortex, pericycle, pith and medullary rays. In the leaves, ground tissue is composed of thin-walled chloroplast containing cells collectively called ‘mesophyll’.
The vascular tissue system:
It is composed of complex tissues, namely xylem and phloem, which together form vascular bundles. In the vascular bundles of dicotyledonous plants, cambium is present between the phloem and xylem. Due to the presence of cambium, such vascular bundles can form secondary xylem and phloem tissues and are known as ‘open vascular bundles’. On the other hand, in monocotyledonous plants, vascular bundles do not contain cambium. They are unable to form secondary xylem and phloem tissues and are thus called ‘closed vascular bundles’.
There are two types of vascular bundle arrangements – radial and conjoint.
The radial arrangement is generally found in roots, where the xylem and phloem in a vascular bundle are arranged in an alternate manner at different radii.
In the conjoint arrangement, found in stem and leaves, the xylem and phloem are situated at the same radius of the vascular bundles.
The conjoint vascular bundles usually have the phloem located only on the outer side of the xylem. In the conjoint open arrangement, the cambium is present between the xylem and phloem while in the conjoint closed arrangement, the cambium is absent.
The plant body has an intricate arrangement of various tissue systems that aid its growth and development.