PLANTS OF THE INDIAN DESERT. 219 



veins except the vertically transcurrent ones in G. niveus, where it 

 forms arcs of palisade cells on the sides of the veins (fig. 298). In the 

 leaf-blade of S. maritimus tubular palisade cells form groups below the 

 epidermis, the adjacent groups being separated by a tissue of loosely 

 arranged arm-palisade cells which are afterwards destroyed to form 

 cavities alternating with the veins. In C. rotundas there is a tissue of 

 thin-walled parenchymatous cells occurring below and between the 

 veins and similarly forming cavities below the stomata. Cells above 

 the zone of the palisade tissue in species of Cyperus and those on either 

 side of the veins in S. maritimus form the aqueous articulation tissue 

 with a two-fold function, that of collecting water and that of 

 regulating the upwards curving movements of the leaf-halves. The 

 abundance of the aqueous articulation tissue in species of Cyperus 

 can be accounted for by the abundance of veins. In F. tenera leaf- 

 blades are thin and the veins together with their girders of palisade 

 cells and a few secretory cells occupy the whole space between the 

 epidermis on either side. 



In the leaf- sheaths of S. maritimus palisade cells form small 

 groups on the sides of the isolated sub-epidermal girders. The veins, 

 except those at the angular portions, are separated by a tissue of stel- 

 late cells which, later on, are destroyed and cavities are formed. The 

 occurence of these lysigenous cavities in abundance is the result of 

 the water-supply being insufficient to maintain all the tissues in a 

 fully developed leaf-blade. The thin-walled parenchymatous cells do 

 not form any important tissue ; they, therefore, lose their turgidity, 

 shrivel up and are destroyed to form cavities. 



Internal glands are represented by numerous secretory cells with 

 tanniniferous contents near the veins in the leaf-blades and leaf- 

 sheaths and in neighbourhood of vascular bundles in the axis. A few 

 secretory cells occur in the ground tissue of the axis. 



Veins are numerous and are confined to the lower half of the 

 mesophyll except in F. tenera and S. maritimus,' in which they are 

 placed in the middle of the mesophyll. Larger veins usually alternate 

 with smaller ones and when veins occur in two planes those in one 

 plane alternate with those in the other (fig. 300). This gives rigidity 

 to the leaf-blade and accommodates a larger number of veins in a small 

 space of the lower half of mesophyll. In species of Gyperus and in 



F. tenera smaller veins are enclosed in two rings, the outer one of 

 stereome and the inner one of green sheath-cells. Larger veins are 

 enclosed in a single outer ring of stereome and the inner ring is 

 represented by two arcs of sheath-cells. Some of the larger veins in 



G. nivens are vertically transcurrent above and below by stereome 

 bundles (fig. 298). In F. tenera a single large vein occurs in the 



