PLANTS OF THE INDIAN DESERT. 35 



INTRODUCTION. 



To one who has the opportunity of visiting the Desert, the 

 flora will at once appear striking and in many respects different 

 from that inhabiting other more favoured provinces. The general 

 barrenness of the country, the dry practically rainless climate and 

 the scorching heat of the sun, not to omit the general susceptibility 

 to the wind, have so modified the external organs of the desert 

 plants, that it is but natural to conclude that the internal structure 

 and the processes that are going on in the plants must in like way 

 have been acted upon by these external factors. 



Physical Aspects of the Indian Desert. 



Some information about the topography, geology and the meteo- 

 rological data of the area under consideration will elucidate the remark. 



(a) Topography. — The portion of the Great Indian Desert, under 

 consideration, is included in the two states Jodhpur and Jaisalmer and 

 forms a square having the four towns Jodhpur, Bap, Jaisalmer and 

 Balmer at its four angles. The general aspect is that of a succession 

 of dry undulating plains and rolling sand-dunes of all sizes and shapes 

 varying from 2 to 3 miles in length and 20 to 400 ft. in height. It 

 is sparingly dotted with low bushes and isolated tufts of curious- 

 looking plants, relieved here and there by green succulents and 

 candelabra-like Euphorbias. Shifting sands are common and con- 

 tinually changing in size and shape. Villages are few and far 

 between and consist of a few huts located round a well or a tank. 



{b) Geology. — The region under survey is covered for the most 

 part with wind-blown sand-dunes which are of the transverse type, 

 i.e., have their longer axes at right angles to the direction of the pre- 

 vailing winds. From beneath these, rocks of earlier age crop out as 

 isolated hills. Near Jodhpur the oldest rocks are schists belonging to 

 the Aravalli system. These are overlain by rocks of volcanic origin, 

 the Mallani series, with conglomerate at the base. Upon these, 

 rest sand-stones of the Vindhyan age. At Balmer the rocks consist 

 of sand-stones, grits and conglomerates containing ill-defined fossil 

 remains. At Jaisalmer sand-stones and lime-stones of Jurassic age 

 occur and Nummulitic rocks are common. 



(c) Meteorological data. — It should be noted that the year 1917, 

 the year of the tour, was remarkable on account of the abnormal 

 amount of rainfall. The data of the normal maximum and minimum 

 temperature, relative humidity of the air, rain-fall and number of rainy 

 days are obtained by averaging the observations of 14 years (pp. 3fi & 37). 



(d) General plan. — As regards the general plan and arrangement 

 of the subject the orders have been described in the sequence of 



