(fig. 163) and D. tomcntosa (fig. 186) ; secretion collects below the cuticle which is swollen like a bladder. Besides the biseriate spheri- cal glands, there are other glandular hairs on the axis of P. scncgalcnsis, which are composed of a long, broad, biseriate stalk and of a small biseriate head (fig. 170) ; there are also peculiar hairs with a long multicellular stalk and with a small biseriate head, occurring in groups on the lower side of the mid-rib of the leaves (fig. 169). External glands are more numerous on the lower surface of the bifacial leaves ; they check transpiration by pouring their secretion on the surface. The greater abundance of stomata on the lower surface brings about the development of a dense covering of clothing and glandular hairs on the lower surface.
Structure of the Axis. — The epidermis is one layered, the outer and inner walls being equally thickened. The former are convexly arched outwards. Lateral walls are thin and undulated. Guard- cells are elevated and the front cavity is on a level with the surface in P. angustifolia, E. echinatus, D. tomenlosa (fig. 188) and L. chon- drilloides ; in other members guard-cells are situated in the same plane as that of the surrounding cells and the front cavity is situated in a depression produced by outer thickened epidermal walls. The elevated position of the stomata is necessitated by the abundance of thin-walled cortical parenchyma.
The primary cortex is characterised by the assimilatory tissue formed either of palisade tissue as in L. chondrilloides or of chloren- chyma as in other members. Collenchyma is developed in the ribs of E. echinatus, V. divaricata and D. tomcntosa, while in P. raj put- anae (Fig. 189 CI) there occurs a continuous sub-epidermal ring of collenchyma. Endodermis is differentiated in P. senegalcnsis, E> echi. natas and I). tomcntosa (fig. 189), and it consists of thin-walled tabular cells with perhaps a water-storing function.
The pericycle consists of a more or less composite ring of groups of stone-cells with small lumina in all members except in E. echinatus, where it is formed of stone-cell groups separated by bast fibres (fig. 182). The arrangement of stone-cell groups in D. tomcntosa is isobi- lateral, larger groups being developed on the side exposed to the direc- tion of the prevailing winds.
