PLANTS OF THE INDIAN DESERT. 73 



in P. irregularis it is single layered on a small portion on one side, 

 indicating the prostrate or inclined nature of the axis, sclerenchy- 

 matcus pericycle being least developed on the lower side of the axis. 

 There are thin-walled colourless cells between and on either side 

 of some of the groups of bast-fibres in P. irregularis ; they may 

 occasionally serve as acqueous cells. 



The wood is composite with vessels uniformly distributed in the 

 interfascicular wood-prosenchyma. In P. irregularis the vascular ring 

 is narrowed on the side where the sclerenchymatous pericycle is 

 reduced ; there are very few vessels in this portion which is mostly 

 occupied by interfascicular wood-prosenchyma. This may be ac- 

 counted for by the fact of less vigorous functional activity on the 

 lower side of the prostrate or inclined axis of P. irregularis. Vessels 

 are large and few and are arranged in incomplete rows. Medullary 

 rays are usually uniseriate and numerous. 



The pith is composed of very thin-walled cells. 



CARYOPHYLLACE.\E. 



Polycarpaea corymbosa Lam. — Figs. 42, 43, 44. Epidermal 

 cells of the leaves with outer walls thickened and papillose. Guard-cells 

 accompanied by subsidiary cells. The front cavity greatly depressed 

 in the axis. Mesophyll isobilateral. Abundance of cells with tannini- 

 ferous contents characteristic of the leaf and axis. Oxalate of lime 

 in the form of clustered crystals. Assimilatory tissue in the axis 

 formed of chlorenchyma. Pericycle composed of large groups of 

 stone cells with a sclerenchymatous tissue on its outer side. Wood 

 composed of large xylem bundles separated by strands of tissue, 

 resembling medullary rays, continuous with the outer sclerenchyma. 

 Large water-storing tracheids with reticulate markings in the xylem 

 bundles near the medullary-ray-like strands. Pith formed of thick- 

 walled cells. 



Structure of the Leaf : — The epidermis consists of polygonal cells 

 with outer walls thickened and papillose (tig. 42). The cuticle is 

 smooth. The lateral and inner walls are thin and the former are wavy. 



The stomata occur on both the surfaces and the guard-cells are 

 accompanied by subsidiary cells. The guard-cells are elevated and 

 the front cavity is placed in a depression formed by the papillose 

 outer epidermal walls. The stomata on the axis are greatly depressed, 

 as the guard-cells are situated below the surrounding cells (fig. 43). 

 In addition to the subsidiary cells, there are sometimes found one or 

 more cells clasping the guard-cells on one or both sides of the stomata 

 on the axis (fig. 43). 



The mesophyll is isobilateral and is composed of palisade tissue. 



1016—10 



