Inge 
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 Arbuscula 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); Lycium sp. 
and Reaumuria 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 directiom 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 
