The pith is composed of thin-walled cells and is characterised in A. baccifera by small groups of thick-walled cells, having the same size and shape as that of the pith cells. These cells form probably the strengthening tissue of the pith.
ONAGRACEAE
Trapa bispinosa Roxb.— Fig. 127. Leaf only.— Epidermis formed of tabular cells with outer walls very greatly thickened on the upper surface. Stomata occurring only on the upper surface. Clothing hairs in the form of uniseriate trichomes and found only on the lower surface. Mesophyll formed of palisade tissue on the adaxial side and of spongy tissue on the abaxial side. Spongy tissue characterised by large intercellular spaces. Oxalate of lime found in the form of numerous clustered crystals. Veins embedded and not provided with bundle-sheaths.
Structure of the Leaf. — The epidermal cells are tabular and thin-walled on the lower surface. The outer walls of the epidermal cells on the upper surface are very greatly thickened. The lateral walls are straight. Stomata occur in large number only on the upper surface and are surrounded by ordinary epidermal cells. Guard- cells are situated in the plane of the surrounding cells and the front cavity is on a level with the surface. Closure of the stomata seems to take place by the well developed cuticular ridges of the guard-cells (fig. 127).
Hairy covering consists of long uniseriate thin-walled trichomes occurring only on the lower surface (fig. 127). External glands are not found.
The mesophyll is composed of long-celled palisade tissue on the adaxial side and of spongy on the abaxial side which is characterised by large intercellular spaces. The system of large intercellular spaces helps the leaves to float on the surface of water.
Internal secratory organs are not found. Oxalate of lime occurs in the form of numerous clustered crystals in the spongy tissue. The cells containing clustered crystals are arranged in small groups projecting into the intercellular spaces. Larger clustered crystals are isolated and occur in the palisade tissue.
The veins are embedded and are not enclosed in bundle-sheaths.
CUCURBITACEAE
Momordica dioica Roxb.— Figs. 128, 129, 130. 131. Leaves membraneous. Guard-cells elevated. Mesophyll formed of short- celled palisade tissue on the upper side and of spongy tissue on the lower. Trichomes on the leaf provided with subsidiary cells not
