96 



the barchans, but they are differently developed, the trees 

 and shrubs being weaker while the herbs are stronger. 

 Another part of the vegetation consists of more exacting 

 species including halophytes which grow in the valleys. 



I look on the vegetation of the Hummock-desert as a 

 sub-formation of the formation of the Sand-desert. 



The vegetation is richer than on the barchans. The desert 

 switch-plants are closer together than on the barchans, but 

 they are smaller and generally take the form of shrubs, not 

 trees. In such a Hummock-desert I found the average height 

 of the bushes to be 2 metres and the distance between them 

 from 7 to 20 metres. The conditions for germination are better, 

 hence the greater density; on the other hand that accelera- 

 tion of growth brought about by drifting sand is lacking, 

 hence the smaller size. 



The Sand Acacia (Ammodendron) is rarer here, while of 

 general occurrence are Salsola Arbusciila and several species 

 of Calligonum (these are difficult to determine without fruits). 

 Amongst other plants present are Saxaul, Eremosparton, Smir- 

 nowia , bushes of Astragalus, and Nitraria Schoberi (which 

 sometimes causes the formation of small dunes); Lijcium sp. 

 and Reanmuria oxiana are both halophytic bushes, generally 

 occurring on clay, but also found in the lower parts of the 

 sand-desert. Tamarisks occur on peculiar knolls of stratified 

 sand which originate as follows: every year the Tamarisks 

 cast numerous small twigs and flower-stalks over which a layer 

 of sand drifts next summer ; then comes another layer of bran- 

 ches and so on. The layers are generally exposed all the 

 way round, and the slopes of the hill are more or less per- 

 pendicular. These low hills are generally circular, or the 

 larger ones are elongated in the direction of the prevailing 

 wind; their height is 2 4 metres. The bushes on them vary 

 from half a metre to 2 metres high, and frequently have old 

 and thick roots, which are exposed where the wind has de- 

 stroyed the hill. On the other hand, hills of this kind may 

 be buried in blown sand. 



These Tamarisk-knolls are presumably remains of a 

 former continuous tract of sandy soil now blown away ex- 

 cept where the roots and shoots of the Tamarisks have 



