Page:The Journal of Indian Botany.djvu/534

102 THE JO0KNAL OF INDIAN BOTANY. occurs on the upper side ; and there is an extensive tissue, occupying two-thirds of the portion of mesophyll, of thin-walled polygonal cells faintly green in colour and partly functioning as an aqueous tissue.

Internal secretory organs are represented by epidermal cells, with tanniniferous contents, of the leaf and axis of B. diffusa and B. elegans.

Oxalate of lime, besides being deposited in the form of granules in outer epidermal walls, occurs in the form of bundles of raphides and of crystal sand in mesophyll of leaves and in the primary cortex of the axis of all members. Granular deposits in the outer epidermal walls are abundant and give to the surface of the leaf and axis a whitish appearance, which besides protecting these succulent plants from being easily devoured by animals on account of the acrid taste, reflect light and arrest transpiration.

The veins are embedded and are enclosed in thick-walled green bundle-sheaths in B. diffusa and B. elegans. Bundle-sheaths are not found in B. verticillata.

The hairy covering consists of glandular hairs and occurs on the axis and both the surfaces of the leaf of B. diffusa and B. elegans. The glandular hairs (tig. 268) are composed of a uniseriate long stalk and of an elongated and ellipsoidal head. The cells of the stalk become narrower towards the apex. External glands are not found in B. verti- cillata.

Structure of the Axis. — The epidermal cells in B. diffusa are poly- gonal, while those in B. verticillata are tabular. The outer walls are thickened, convexly arched outwards and granulated with deposits of calcium oxalate. The primary cortex (fig. 269) is composed of collen- chyma on its outer side and of chlorenchyma on the inner side. The endodermis is distinct and is composed of tabular cells which are thick-walled in B. diffusa and B. elegans.

The pericycle consists either of isolated bast fibres as in B. diffusa and B. elegans or of a loose ring of bast fibres in B. verticillata. The wood is characterised by the occurrence of two rings of xylem bundles. The primary bundles are small and are embedded in a thin ring of interfascicular wood prosenchyma which is composed of cells with thick walls and with small lumen. The secondary bundles are irre- gularly arranged in the pith and are separated by broad strips of thin- walled parenchymatous cells, resembling medullary rays. Soft bast is found in groups on the outer side of xylem bundles.

The pith tissue between the primary and secondary vascular bun- dles consists of cells with thickened and lignified walls and filled with granular contents, and that between and enclosed by the secondary vascular bundles is formed of very thin-walled cells.