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 usceptibility 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 meteorological 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 continually 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 prevailing 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
