There are three types of tissue systems in plants, viz. epidermal tissue system, ground tissue system and vascular tissue system.
The outer covering of the plant body is formed of epidermal tissue system. The epidermal tissue system is composed of epidermal cells, stomata and edidermal appendages (trichomes and hairs).
Epidermis: The outermost layer of the primary plant body is called epidermis. The epidermis is made up of elongated, compactly arranged cells. These cells form a continuous layer. There is usually a single layer of cells in the epidermis. The epidermal cells are parenchymatous. In these cells, a small amount of cytoplasm lines the cell wall and a large vacuole is present. The outer surface of epidermis is usually covered with a waxy cuticle. The cuticle prevents the loss of water. However, cuticle is absent in roots.
Stomata: The minute pores present in the epidermis of leaves are called stomata. A stoma is composed of two guard cells; which are bean-shaped. The guard cells are dumbbell-shaped in grasses. The outer wall of guard cells is thin and the inner walls are highly thickened. Chloroplast is present in the guard cells. The guard cells regulate the opening and closing of stomata. A few specialized epidermal cells may be present near the guard cells. These specialized cells are called subsidiary cells. The stomatal aperture, guard cells and subsidiary cells together make the stomatal apparatus. Transpiration and exchange of gases are regulated by stomata.
Hairs: The root hairs are unicellular elongations of the epidermal cells. They absorb water and minerals from the soil. The epidermal hairs on stem are called trichomes. The trichomes are usually multicellular. They can be branched or unbranched. They can be soft or stiff. The trichomes prevent water loss through transpiration. Some of the trichomes may be secretory.
Tissues; other than epidermis and vascular bundles; make the ground tissue. The ground tissue is composed of simple tissues. Parenchyma is usually present in the cortex, pericycle, pith and medullary rays; in the primary stems and roots. The ground tissue in leaves is composed of thin-walled, chloroplast containing cells. This is called the mesophyll; in leaves.
The vascular tissue system is composed of complex tissues.
Vascular Tissue in Dicot Stem: In dicotyledonous stem, cambium is present between phloem and xylem. Because of the presence of cambium, the vascular bundle in dicot stem is capable of forming secondary xylem and phloem. Hence, these are called open vascular bundles.
Vascular Tissue in Monocot Stem: Cambium is absent in monocot stem. Hence, secondary xylem or phloem is not formed in monocot stem.
Radial Arrangement of vascular bundle: This type of arrangement is present in roots. In this case, xylem and phloem are arranged in an alternate manner on different radii.
Conjoint arrangement of vascular bundle: This type of arrangement is present in stems and leaves. In this case, the xylem and phloem are on the same radius. In this case, the phloem is usually located on the outer side of xylem.
Copyright © excellup 2014