PLANTS OF THE INDIAN DESERT. 101
prosenchyma is extensive and is composed of thick- walled cells with small lumen. The medullary rays are uniseriate and numerous. Wood parenchyma is little developed.
The pith consists of thin-walled cells.
NYCTAGINACEAE.
Boerhaavia diffusa L— Figs. 268, 269. Upper epidermis of cubical cells. Lower epidermis of tabular cells. Epidermal cells of the leaf and axis with tanniniferous contents. Palisade tissue on the upper side and round the veins. Aqueous tissue below lower epidermis. Veins with thick-walled green sheaths. Glandular hairs with a uniseriate stalk and with an ellipsoidal head. Epidermal cells of the axis polygonal. Pericycle formed of isolated bast fibres. Endodermis of thick-walled tabular cells.
Boerhaavia verticillata Puir.— Epidermis of both sides of tabular cells. Mesophyll bifacial. Glandular hairs absent. Veins without bundle-sheaths. Pericycle forming a loose ring of bast fibres. Endodermis of thin-walled tabular cells.
Boerhaavia elegans Chois.^ Fig. 270. Specific characters al- most the same as those of B. diffusa. From the point of view of anatomical characters it should belong to the same species.
Structure of the Leaf. — The epidermis on the upper side of B. diffusa and B. elegans consists more or less of cubical cells with the outer and inner walls a little thickened. The latter are convexly arched outwards. The lower epidermis in B. diffusa and B. elegans and the epidermis on both sides in B. verticillata consists of tabular cells with the outer walls thickened and convexly arched outwards. The lateral walls are thin and straight in all members. The granula- ted appearance of the outer walls is due to the deposits of crystaline granules of calcium oxalate ; and the granulated walls are covered by a thick cuticle. The occurrence of deposits of calcium oxalate in the epidermal walls of these succulent plants protects them from desiccation as well as from being easily devoured by animals on account of the acrid taste.
Stomata occur on both the surfaces and are surrounded by ordinary epidermal cells. The guard-cells are situated in the plane of surrounding cells and the front cavity is placed in a depression formed by the upper portions of surrounding cells.
The mesophyll in B. diffusa and B. elegans is composed of palisade tissue on the upper side and about the veins and of a tissue of thin-walled colourless polygonal cells with probably a water- storing function on the lower side. In B. verticillata palisade tissue
