Simple tissues are a collection of similar cells that perform a common function. These tissues are of 3 types.
(a) Parenchyma (b) Collenchyma (c) Sclerenchyma.
In parenchymatous tissue all cells are isodiametric and thin walled. The cells may be oval, circular or polygonal with intercellular spaces. They are living. This tissue is present in all organs of the plant e.g., roots, stems, leaves, flowers, fruits and seeds.
The main function of this tissue is storage of food.
2. In fleshy stems and Fig. 15.8 Parenchyma. leaves parenchyma cells function as water storage tissue e.g., Opuntia, ^Euphorbia,
3. In hydrophytes large air cavities are formed in between cells of these tissues, which are filled with air. This types of parenchymatous tissue is called aerenchyma. The plants become buoyant due to the presence of air and also gaseous exchange is facilitated.
4. Some times in parenchymatous cells develop chloroplasts. Such parenchyma is called chlo-renchyma and such cells participate in photosynthesis.
5. Some times such cells start to divide e.g., Cork cambium root cambium, and help the plant in secondary growth, formation of cork and healing of wounds.
Collenchymatous cells are longer than parenchyma cell.. They are living. The intercellular spaces in this tissue are absent because in intercellular spaces at the corner of cells thickenings of cellulose and pectin develop due to which the cell wall become rigid and thick at corners. Sometimes collenchyma develops chloroplasts. Collenchyma is usually found as 3-4 layered hypodermis of herbaceous dicotyledonous stem. They are absent in monocot stems.
1. It provides mechanical support to the organs and due to its peripheral position in stems it resists bending and pulling action of wind.
2. When chloroplasts are present it takes part in photosynthesis.
This tissue differ from previous tissues in possessing thickened secondary walls which may or may not be lignified. They are of various shapes and sizes when mature. They are dead. They are of two types :
1. Sclerenchymatous fibres 2. Stone cells
These are discussed in detail.
1. Sclerenchymatous fibres :
Fully developed fibres are long and tapering at the ends. In some cases the cellwall becomes so much lignified that the lumen is greatly reduced.Wall has small circular pits. The normal length of fibres is 1-3 mm but in jute and Boehmeria their length may be from 20-550 mm.
The fibres are present in hypodermis of monocot stem,in pericycle of many dicots, in secondary wood, in vascular bundle sheath in monocot stems and hypodermis of many leaves.
The main function of sclerenchyma is mechanical strength.
2. Stone cells (Sclereids) :
They are lignified, extremely thick walled so that the lumen of the cells is almost oblitrated and may be spherical,oval,cylindrical,T- shaped and even stellate. They are generally found in hard parts of the plant e.g. endocarp of walnut and coconut. They form part of seedcoat in some members of leguminoseae.