79 



are mostly found in the northwestern part of the lowland. 

 They are comparatively well covered with switch-like desert- 

 shrubs, Carex physodes, etc., and the sand-drift is of no great 

 importance. 



5. "Dunes", or recent accumulations of shifting greyish 

 or white sea-sand occurring mostly along the coast of the 

 Caspian, and generally arranged in chains which follow the 

 direction of the wind. 



Dune -chain sands and recent dunes, both formed by 

 white or grey sea-sand, 1 have not seen, hence they are here 

 left out of consideration, and only the different forms of 

 inland-sand are dealt with. 



The first point to be considered is sand as a soil for plants 

 in comparison with clay. 



In dry countries sand is, in some respects, more favour- 

 able to vegetation than clay. 1 ) Water is quickly absorbed so 

 that it has no time to evaporate. Less water will evaporate 

 from a rough, coarse-grained surface of sand than from an 

 even, fine-grained surface of clay. Because of the slight water- 

 holding capacity of sand, the water is carried to greater depths, 

 whence it does not rise easily to the surface owing to the 

 poor capillarity of sand. The evaporation-surface of the water 

 will therefore be situated down in the earth where it is pro- 

 tected by the overlying drier layers of. soil (comp. LIVINGSTON 

 1906). Deep sand is a soil which suits plants with very long 

 roots. The switch -shaped trees and shrubs generally occur 

 here. 



Though sand can absorb much less water than clay 

 (14,3 per ct. of dry weight, loess 59,3 per ct. according to 

 CLEMENTS, p. 34), almost all the water absorbed is available 

 for the plants: "Echard" is only 0,3 per ct., "Chresard" 14 

 per ct. (comp. above p. 58). The figures vary of course some- 

 what according to the properties of the sand, especially the 

 size of the grains (LIVINGSTON 1905), but I am not aware 



!) Fitting (p. 251) also finds that the sand-plants of the Sahara have 

 a lower osmotic pressure than the plants of the stone-deserts even when 

 they belong to the same species. 



