TUE FLORA OF THE INDIAN DESERT. 507 



The opposite side is steep and rough. The most interesting phenomenon, how- 

 ever, IS caused by two very hard, flint-hke yellowish nodules enclosed 

 in the rock. These show hardly a trace of erosion, and remain sticking out from 

 the pohshed surface of the rock, causing an eddy to be formed in the sand-laden 

 stream of air. The result is the formation of a smooth cup in the rock in front 

 and on the sides of each nodule, and a slopmg central longitudinal ridge behind. 

 This is precisely the same phenomenon as may be observed when an obstacle 

 is placed in a hard snowdrift, which is being acted upon by dry, wind-driven 

 snow. 



Another characteristic action of the wind on the surface of the sand is the 

 sorting of the sand particles, resulting in the formation of wind-ripples. PI. II 

 B. shows a rippled surface of a dune-crest near Loharki. Where these ripples 

 are formed they are in a constant forward motion as long as the wind is strong 

 enough. Under these circumstances, germination of seeds is of course impossi- 

 ble, and the area shown in the photo is totally devoid of vegetation. 



The sand formation includes some of the most characteristic associations of 

 the region. It is evident that on a rapidly shifting dune, hardly any plants 

 can obtain a footing (Plates I-A., II-B.). Occasionally, however, even in such 

 places, some of the seeds carried along with the sand are able to germinate 

 during the rainy period, when the sand is less mobile. The character of the 

 seedling will then determine whether the plant is able to develop further, after 

 the sand has become drier. Plate II-A. shows part of an almost bare dune-area, 

 facing NE near Loharki village, on which scattered specimens of the following 

 plants were noted : Calotropis procera, Leptadenia spartium, Crotalaria burhia, In- 

 digofera argentea, Aerua pseudo-tomentosa, Citrullus colocynthis, Farsetia jacque- 

 montiana. Eleusine sp., Panicum turgidmn. This list gives a fair idea of an early 

 stage in the colonisation of a shifting dime. Of the plants mentioned, Calotropis 

 procera is perhaps best fitted to survive, being a rapidly growing, coarse shrub 

 very indifferent as to habitat. Plate XXV-A. shows the western slope of a dune 

 near Phalodi, wdth a pure association of this shrub. Indigojera argentea is also 

 a typical dune pioneer (see Plate VII-B., showing the eastern slope of the north- 

 end of the above mentioned dune, colonised exclusively by this plant). It owes 

 its pecuhar fitness as a sand-plant to its mode of growth. The thick, woody 

 stem protrudes from the sand a few inches, and supports on its top a crown of 

 twiggy branches, spreading out horizontally. Where, as in the above habitat, 

 the individual plants grow tolerably close, the flat crowns form a layer, preventing 

 the wind from reaching the surface and stirring up the sand to any marked 

 degree. Even if the sand should be disturbed, the naked part of the stem will 

 generally prove tail enough to protect the green crown from being swamped. 

 In addition, the root is a strong, very long tap-root, woody like the stem, and 

 capable of descending to a great depth in search for water. 



Where the sand has spread out over a larger, horizontal area, it is soon colonized 

 by the two species of Aerua : Ae. tomentosa and Ae. pseudo-tomentosa. Such 

 an association close to Sodakoer village is shown in Plate XXII- A., and is gener- 

 ally a very noticeable feature in the landscape, on account of the greyish white 

 appearance of the shrubs, as contrasted with the brownish tints of the surround- 

 ing gravel area. In the instance in question, the association contains islands 

 of shghtly elevated sand-mounds, bearing famiUes of Capparis decidna their 

 elevated position bemg due to the capacity of the lower, rambling branches of 

 the shrubs to retain blown sand. These famiUes were attacked by the root- 

 parasite Cistanche tubulosa, now in flower, which in Plate XXII-B. may be seen 

 breaking through the compact top-layer of damp sand. 



On Plate XXXII-A. is shown the edge of the dune area close to Loharki village, 

 which is figured on Plate 11. Here a family of Calotropis procera has succeeded 

 in colonising not only the thin layer of sand on the plain below, but also tlie 



