There are two types of tissues. The first is simple to show or a homogeneous tissue of having only one type of cells. Another complex type of tissue is heterogeneous and comprises more than one type of cells functioning as a unit xylem and phloem are complex tissue. They are helpful in transport of water, salts and food material to the various parts of the plant body. Phloem is main tissue for conduction of food material.
These are homogeneous in nature and are composed of structurally and functionally similar cells. The simplest tissues are parenchyma, callenchyma and sclerenchyma.
Parenchyma: Parenchyma is the most common tissue with which is morphologically and physiologically unspecialized, forms the framework of all plant organs and tissues like cortex, pith, mesophyll of leaf and floral parts. Parenchymatous cell are usually isodiametric but may be elongated or even lobed as in the mesophyll tissue of the leaves. These may either be packed or show small and inter-cellular spaces. The plasmodesmata (thread like cytoplasmic strand, running from one cell to other) are commonly present. When the Parenchy-matous cells are exposed to light, these develop chloroplast in them and such a tissue is known a chlorenchyma. The cells of parenchyma are involved in the various physiological activities like photosynthesis, assimilation, storage, secretion, excretion etc.
Collenchyma: Collenchyma is another simple tissue composed of more or less elongated cells with primary, non-lignified cell walls. Their cell wall is the most distinctive feature which is characteristically unevenly thickened. The wall thickening is primary in nature and is composed of cellulose, hemicellular and pectic materials with a high percentage of water. The thickenings may be primarily at the corners or angles of the cells. These have vavcolated protoplast and occur characteristically in the hypodermis of herbaceous dicots, either as a homogenous layer or in patches. These cell constitute an effective mechanical tissue and provide elasticity and support to the growing organs.
Sclerenchyma: Sclerenchyma is third simple tissue meant specially mechanical support. These are considerably thick wall and lignified with simple or bordered pits in their wall and are characterized by the absence of living protoplast. On the basis of variation in form, structure, origin and development, these may be either fibers or scleride. The former are pointed and niddle like. The fibers occur in groups as sheets or as cylinders in various parts of the plant body. Sclereids are very thick walled, hard and strongly lignified. They are mostly isodiametric, polyhedral, short and cylindrical. These are dead cells with very narrow cell cavities (lumen) due to excessive thickness of the cell wall.