80 



that investigations on these conditions have been made in 

 Transcaspia. 



Where the sand forms a stratum over the loess it be- 

 comes of special importance. The water from precipitation 

 will be let down through the sand into the upper layers of 

 loess, whence it cannot evaporate because protected by sand, 

 and yet it is still available for the roots of plants, if the sand 

 is not too deep. The natural conditions are here specially 

 favourable, and it is on sand over loess that the most lux- 

 uriant desert-vegetation is found during summer (see chap. 11). 

 These conditions are now successfully imitated by man. In 

 the dry parts of North America "dry farming" is conducted, 

 the principle being that the subsoil, by the aid of special 

 implements, is always kept solid, so that it can raise the 

 water, while the surface-soil is kept loose so that it can pro- 

 tect the subsoil and itself lose the least possible amount of 

 water through evaporation (see MATENAERS). 



Though sand is more easily leached than clay, the under 

 ground water in the sand-desert is almost always salt, and 

 gypsum crystals frequently occur in quantity at a depth of 

 1 /a 1 metre (PALEZKIJ p. 36). Many of the sand-plants are 

 also halophytic in structure. 



Where no water is present the desert-sand is an exceed- 

 ingly hot soil, all the more as it is not white but brown. 

 The expedition recorded 53 C. on a summer's day just below 

 the surface, but even higher temperatures might certainly 

 be found. 



Sand is more unfavourable to vegetation than clay, in 

 this respect, that the sand is moveable. Sometimes the roots 

 of the plants are laid bare, sometimes aerial shoots are buried, 

 either of which conditions may kill the plant. The drifting 

 sand-grains may also bruise young or unprotected plant-tissues 

 and in this way cause injury. This has already received 

 attention in the literature on European dunes, e. g. WAR- 

 MING 1909. 



The following description of the vegetation of the Sand- 

 desert deals first with the more shifting deserts, afterwards 

 with the stationary types. The desert is described in its 



