A tissue consists of cells of only one or several types, having a common origin and performing an identical function (Fig. 98).
(a) Meristematic:
These are small, thin- walled cells, devoid of any central vacuole and situated at the growing points of stems and root. Their chief function is mitosis.
(b) Protective:
The cells of these tissues are found on the surface of roots, stems and leaves. Top and bottom surface of these flat cells are parallel and their sides are irregularly arranged.
(c) Parenchyma:
These are large, thin-walled, living cells, each having a nucleus and central vacuole. Intercellular spaces are present in between these cells. Parenchymatous cells, present in the areas not exposed to light, possess leucoplast. Parenchyma cells, specialized for the process of photosynthesis, are called chlorenchyma.
(d) Collenchyma:
These cells have thick secondary wall, especially well developed at their corners. Mechanical support is provided for the plants by these cells.
(e) Sclerenchyma:
Very thick secondary walls are present in the form of a thick uniform layer around the entire margin of these cells. Generally, after the formation of secondary cell wall, the protoplasts of these cells die.
(f) Xylem:
It consists of several cell types like xylem vessels, xylem tracheitis. Vessels are absent in the xylem of Gymnosperms. Vessels have thick secondary cell walls which are not deposited in a uniform layer but usually thickened in the form of spiral bands. Spiral bands are not present in the tracheids. Tracheitis are tapered at the ends and interconnected by means of pits. Xylem develops into wood.
(g) Phloem:
Important cells of this mixed tissue are sieve tubes and companion cells. Sieve tubes have a perforated end walls. Nucleus is absent in the sieve tubes at maturity but present in the companion cells. Chief function of the phloem is the transport of food throughout the plant.