— 58 — 



opposed to Echard, the percentage retained by the soil and 

 which the roots cannot absorb even if the plant wilts. Ac- 

 cording to Clements (p. 31) loess can absorb 59,3 per ct. (of 

 its dry weight), and of these 49,2 per ct. are Chresard, 10,i 

 per ct. Echard. 



In the case of clay Clements gives 9,s per ct. Echard, 

 Sachs has 8 per ct. (Vorlesungen, 2. Aufl. p. 239). Sand can 

 absorb much less water, but on the other hand, almost all 

 the water is available for the plants: the Echard is very 

 small (Comp. E. Gain 1895). 



The capillarity of loess is considerable, so that where 

 ground- water occurs it is able to raise this higher than sand, 

 although the rale of movement is slower. On the one hand 

 this process brings the deeper-lying water within reach of 

 shorter roots, but on the other hand it promotes the evapora- 

 tion of ground-water in loess as compared with that in sand. 



Since the surface of loess is even and fine-grained, more 

 water will evaporate from it than from a sand-surface (Ra- 

 mann p. 262). Another factor which promotes this is that 

 loess has a dark colour and is therefore strongly heated by 

 the sun. Middendorff records the following surface-tempera- 

 tures on a sunny day in May. On loess 62** C, on a white 

 salt-incrustation 45° C. The lower temperature of the salt- 

 incrustation is due to combined evaporation and reflection. 



Thus it will be seen that loess under dry conditions is 

 as unfavourable to vegetation as it is favourable when water 

 is present. 



The maximum of precipitation (p. 17) for the areas we 

 are considering occurs during winter or spring, whereas the 

 summer is practically rainless, but very hot. The plants of 

 the clay-desert which live through the summer in a vegeta- 

 tive condition cannot be adequately supplied by the small 

 proportion of the comparatively limited spring rain which 

 remains in the clay-soil. These plants, the Summer -PI ants, 

 must therefore supplement their water-supply from the saline 

 ground-water. Thus one finds that in the true dry Clay- 

 deserts where neighbouring mountains do not make the con- 

 ditions specially favourable, the plants are almost all Halo- 

 phytes. Conspicuous amongst them are many Chenopo- 



