PLANTS OF THE INDIAN DESERT. 63 



jecting mid-rib and by the downwardly curved margins. In this 

 portion stomata as well as hair-like epidermal cells are more nume- 

 rous than on the upper surface. The stomata are thus protected from 

 direct sun light and transpiration is very much checked. 



The mesophyll is isobilateral in E. aspera, 3. rariflorum, H. 

 undulatum and S. pauciflorum ; in E. aspera palisade cells on the 

 upper side are much longer than those on the lower. It consists of 

 palisade tissue on the upper side and of arm-palisade tissue on 

 the lower in H. supinum, H. panicidatum, T. indicum and A. hispi- 

 dissima. There is a tissue of large thin-walled colourless polygonal 

 cells in the middle of the mesophyll of all members except H. supinum, 

 T. indicum and A. hispidissima. This tissue, which may be termed the 

 middle tissue, seems to partake in the formation of bundle-sheaths. 

 The cells of the middle tissue may hold clustered crystals, or tannini- 

 ferous contents, or they may serve occassionally as water-reservoirs. 



Internal secretory organs are represented by tannin sacs confined 

 to the middle tissue of the mesophyll, or to the bundle-sheaths. The 

 cells of the middle tissue as well as those of the bundle-sheaths in 

 H. rariflorum and E. aspera hold tanniniferous contents which are 

 confined only to the sheath-cells, or to the cells of the middle tissue 

 in II. panicidatum and H. undulatum respectively. 



Oxalate of lime is present in the form of clustered crystals in 

 groups of cells between the middle tissue and epidermis on either 

 side in 3. undulatum (fig. 212). Solitary crystals are found in the 

 pith cells of E. aspera and 3. rariflorum. 



Calcium carbonate is deposited either in the outer epidermal 

 walls as in 3. supinum or in walls of clothing hairs as in other 

 members. Deposits of calcium carbonate give a warty appearance to 

 the surface of the leaf or to the walls of the hairs, and they are the 

 cause of roughness of the leaf and of the axis. 



The veins are embedded in all members except in E. aspera in 

 which they are vertically transcurrent above and below by scleren- 

 chyma. Bundle-sheaths occur round the veins in E. aspera, 3. supi- 

 num, 3. rariflorum, T. indicum and A. hispidissima. The veins of 

 the mid-rib are protected by thin-walled collenchyma which surro- 

 unds them. The vascular bundles are bicollateral in 3. panicidatum. 

 The veins are numerous in all members except in herbaceous 

 members, viz : 3- supinum, T. indicum and A. hispidissima- They 

 anastomose freely and are prominent either on both the surfaces as 

 in 3. rariflorum or only on the lower surface as in E. aspera and 

 3. undulatum- The veins of the mid-rib are prominent on the lower 

 side in all the members. Upper surface of the mid-rib is grooved 

 in S. pauciflorum. 



