ttiANifS OF THE INDIAN DESERT. % 



in size as the vessels and situated on the upper side of the xylem 

 near the medullary-ray-like strands. Interfascicular wood-pro- 

 senchyma is not developed. Wood-parenchyma occurs in groups at 

 the inner margin of the xylem bundles. 



The pith is composed of thick-walled cells. 



PORTULACFAE. 



Portulaca oleracea L. (Leaf only).— Clustered crystals in 

 the leaf small and numerous. 



Portulaca quadrifida L. (Leaf and Axis). Clustered crystals 

 in the mesophyll large and few. 



Structure of the Leaf : — The epidermis is composed of polygonal 

 cells with outer and inner walls thickened. The outer walls are 

 arched convexly outwards and are granulated. Stomata are more 

 numerous on the lower surface. The guard-cells are elevated 

 and accompanied by subsidiary cells. The front cavity is on a level 

 with the surface. The mesophyll is composed almost wholly of 

 aqueous tissue, the vascular bundles of the veins being surrounded' by 

 palisade parenchyma. Internal secretory organs occur neither in 

 the leaf -nor in the axis. 



Oxalate of lime occurs in the form of clustered crystals in the 

 aqueous cells of the mesophyll and in the cortex and pith of the axis. 



The veins are embedded and are not provided with bundle 

 sheaths ; they are surrounded by palisade parenchyma. 



The leaf and axis are devoid of hairs. 



Structure of the Axis : — The epidermal cells are polygonal with 

 outer and inner walls greatly thickened. The outer walls are con- 

 vexly arched outwards and are granulated. The stipular rings of sil- 

 very long hairs reflect light and protect the axis and leaves against 

 intense light and heat. 



The cortex is formed on its outer side of an extensive tissue of thin- 

 walled parenchymatous cells filled with starch granules and on its 

 inner side of chlorophyll containing parenchyma. The cells of the 

 outermost layer of the cortex are collenchymatous. The cortex may 

 occasionally form an aqueous tissue. 



The sclerenchymatous pericycle is not developed. 



The vascular system is composed of deeply placed vascular bun- 

 dles separated by thin-walled uniseriate medullary rays. Vessels are 

 small and are arranged in complete rows. The vascular ring is sur- 

 rounded by green parenchyma. The nearness of the assimilatory 

 tissue to the vascular bundles brings about a quick distribution of the 

 products of assimilation. 



(To be continued) 



