PLANTS OF THE INDIAN DESERT. 195 



a short uniseriate stalk and of an ovoid curved head. Pericycle 

 formed of a composite ring of stone-cells. Axis ribbed. Ribs strength- 

 ened by collenchyma. Assimilatory tissue in the axis formed of 

 chlorenchyma. Wood formed of xylem bundles connected by strands 

 of interfascicular wood prosenchyma. Medullary rays absent. Pith 

 formed of thin walled cells. 



Structure of the Leaf '.: — The epidermis consists of tabular cells 

 which are much larger on the upper surface. The outer walls are a 

 little thickened ; the inner walls are gelatinised, and those of the upper 

 epidermis are greatly arched convexly inwards. The lateral walls 

 are very thin and straight. The gelatinisation of the inner walls of 

 the epidermal cells compensates for the hairy covering which is 

 scanty. The stomata are more numerous on the lower surface, and 

 are surrounded by ordinary epidermal calls. 



The mesophyll is composed of palisade tissue on the adaxial 

 side and of arm-palisade tissue on the abaxial side. Internal secretory 

 organs do not occur in the leaf. They are represented in the axis 

 by secretory cells with tanniniferous contents in the soft bast and 

 pith. Oxalate of lime occurs in the form of numerous clustered 

 crystals near the veins in the leaf. The clustered crystals occur in a 

 layer of cells outside the pericycle and in the pith of the axis. 



The veins are provided with green bundle-sheaths. The leaves 

 are many-ribbed and the veins of the ribs, which are prominent 

 below, are vertically transcurrent above and below by collenchyma. 

 The smaller veins are embedded. 



The hairy covering consists of clothing and glandular hairs. The 

 clothing hairs on the leaf are short, thick-walled, unicellular and with 

 a sharp point. The clothing hairs on the axis are longer with 

 muriculate walls; "the basal portion is divided by two to three cross- 

 walls. The glandular hairs are formed of a short uniseriate stalk 

 and of a small ovoid head which is curved and is divided by horizon- 

 tal walls (fig. 95). 



Structure of the Axis : — The axis is ribbed. The epidermal 

 cells are small and vertically tabular with outer walls greatly thick- 

 ened and convexly arched outwards. The primary cortex is formed 

 of chlorenchyma which is bounded internally by a layer of thin- 

 walled colourless cells mostly containing clustered crystals. Strands 

 of collenchyma are developed in the ribs. 



The pericycle is formed of a composite ring of stone-cells. The 

 wood is composed of xylem bundles connected together by narrow 

 strands of interfascicular wood prosenchyma formed of cells having 

 thin walls and large lumina. The vessels are very large and have 

 simple perforations. Medullary rays are absent. 



