Collenchyma has living cells which possess thickenings at the corners. It is found in few layers below the epidermis of dicotyledonous stem. It is generally absent in monocot stem and leaves. These are mechanical tissues, which also consist of living cells. Intercellular spaces are generally 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. Collenchyma gives strength to the stem. Chloroplast containing collenchyma also manufactures food.
Sclerenchyma is found in hard parts of the plant body. They are very common in roots, stems, leaves and petioles. They may be present in patches, groups or layers. The cells of the sclerenchyma are dead, they are elongated, narrow, and thick walled and lignified. They are pointed at both ends. It gives strength, rigidity and flexibility to the plant body. This consists of fibres and sclereids;
Fires are long, narrow, thick and liquefied cells usually tapering at both ends.
Sclereids cells are normally short with very thick walls, irregular and not tapering at the ends.