PLANTS OF THE INDIAN DESERT. 3 



The epidermal cells of the inid-rib are much smaller than those on 

 the other portions of the leaf-blade. The stomata are usually equally 

 distributed on both the surfaces. Guard-cells are elevated in Pegol- 

 ettia senegalensis (fig. 167), Pulicaria angusti folia, E. echinatus and 

 V. divaricata, so that the front cavity is on a level with the surface- 

 The guard-cells are situated in the plane of the surrounding cells in 

 other members, so that the front cavity is placed in a depression 

 formed by the outer thickened epidermal walls (figs. 174, 184, 190). 

 In Pegolettia senegalensis elevated stomata occur side by with depress- 

 ed ones (fig. 167). 



The mesophyll in D. tomentosa and E. echinatus, is composed of 

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

 abaxial side ; it is isobilateral in V. cinerascens, P. rajputanae and 

 V. divaricata. In other members it consists of arm-palisade cells with 

 horizontally elongated cells in the middle. 



The veins are embedded except those of the mid-rib which are 

 vertically transcurrent above and below by collenchyma. In V. cinera- 

 scens there are a few stone-cells on the upper side of the veins. The 

 veins of the mid-rib are quite prominent beneath and are strengthened 

 on the lower side by arcs of stone-cells. 



Internal secretory organ3 are not found except in P. rajputanae 

 in which secretory cavities occur one on either side of the veins of the 

 mid-rib, with a lining layer of cells and with yellowish contents. 

 Oxalate of lime occurs in P. senegalensis, in the form of bundle of 

 acicular crystals near the veins, in cortex and in pith. 



Hairy covering on the leaf and axis consists of clothing and 

 glandular hairs. Clothing hairs, termed " flagellum-hairs " consist of 

 a uniseriate stalk and of a flagellum-like terminal cell in P. angusti- 

 folia, P. rajputanae, E. echinatus and D. tomentosa ; in E. echinatus 

 the long terminal cell is bent on the stalk as on a hinge. In V. cinera- 

 scens (fig. 164) there are two-armed hairs, consisting of a uniseriate 

 stalk and of a terminal unicellular two-armed cell ; the arms are 

 unequal. The glandular hairs are of various shape. They are placed 

 in epidermal depressions and consist of a short uniseriate stalk and of 

 a large biseriate head in P. senegalensis (fig. 167), P. angustifolia, 

 (fig. 172) and E. erecta. The external glands, in E. echinatus (fig. 181) 

 and V. divaricata, consist of a long uniseriate stalk and of a head 

 irregularly divided ; besides these there are in E. echinatus (fig, 179) 

 spherical glands which are uniseriate, depressed and with thickened 

 and verrucose walls. They are club-shaped in P. rajputanae (fig. 176) 

 and are long-stalked, uniseriate and with the terminal cell much 

 dilated to form the head in V. cinerascens (fig. 165, 166). Uniseriate, 

 spherial and depressed glandular hairs are found in V. cinerascens 



