PLANTS OF THE INDIAN DESERT. 161
Structure of the Leaf. — Epidermal cells are polygonal with outer
walls thickened and papillose on both the sides in C. polygonoides
and on the upper only in 1 P. plebejum. Epidermal cells on the upper
side in the latter are tabular with outer walls thickened and flat.
Large water-storing cells are intercalated amongst ordinary epidermal
cells in P. plebejum. Lateral walls are thin and undulated.
Stomata are surrounded by ordinary epidermal cells and are equally numerous on both the surfaces. Guard-cells are in the plane of surrounding cells and the front cavity is placed in a depression formed either by outer thickened epidermal walls or by the papillae.
Mesophyll is isobilateral in G. polygonoides, while in P. plebejum it consists of palisade tissue on the upper side and of arm-palisade on the lower. There is a tissue of polygonal cells with tanniniferous contents in the middle of the mesophyll in G. polygonoides.
Internal secretory organs are represented in G. polygonoides by polygonal cells in the middle of the mesophyll and by palisade-like cells, in the palisade tissue, with tanninifarous contents. Sheath- cells in P. plebejum hold tanniniferous contents. Oxalate of lime occurs in the form of clustered crystals in the mesophyll of P. plebejum.
Veins are embedded and are enclosed in bundle-sheaths. Sheath-cells in P. plebejum hold tanniniferous contents.
Hairy covering, in G. polygonoides, consists of uniseriate trie- homes which are composed of a basal cell and of a terminal cell with verrucose walls and more or less adpressed. External glands in C. polygonoides are club-shaped and consist of a biseriate short stalk and of a head irregularly divided.
Structure of the Axis. — The epidermis consists of small polygonal cells with outer walls thickened and papillose, thickening being considerable in G. polygonoides. Outer walls are granulated in P. plebeium. Lateral walls are thin and undulated. Some of the epidermal cells in C. polygonoides hold tanniniferous contents. The primary cortex is characterised by an assimilatory tissue of palisade cells. The mechanical tissue is represented by sub-epidermal strands of collen- chyma and of stone-cells in the ribs of P. plebejum and C. polygonoid- es respectively. Along almost one-third of the circumference of the axis, cork is developed below epidermis. This suggests that the axis is greatly inclined and that cork is developed on the upper side which is exposed to the sun. Endodermis is differentiated and is characterised by tanniniferous contents in the species of both the genera. The pericycle is composed of small groups of stone -cells (figs. 292, 293). Stone-cell groups in C. polygonoides are radially much elongated. Cells interposed between stone-cell groups are
