— 103 — 
whereas on the more stabilised sands these give place to 
perennials and undershrubs. This is only natural, for if the 
perennials have not, like Aristida, unusual powers of resisting 
sand-drift, they will sooner or later be smothered by a shift- 
ing barchan and very few, if any, will produce seeds during 
the first year; on the other hand, the annual plants with a 
shorter growth-period will have a greater chance of surviving 
and ripening seeds. 
The difference in the numbers of annuals in Hummock- 
deserts and Desert-plains appears to be due to the annual 
halophytic Chenopodiaceae which occur in the depressions 
of the Hummock-desert, but seem to be of less importance 
in the Desert-plains. 
Finally, attention is directed to the trees of the desert 
(the switch trees and shrubs) which as emphasised in the 
preceding pages, play the most promiment part and attain 
the richest development in the shifting desert, while they 
deteriorate where the sand is stable. Sand-drift seems to be 
a condition essential for vigorous growth in their case and 
also with Aristida pennata. 
Making a mental comparison between the sub-formations 
of the sand-desert described, we see that they have many 
features in common both floral and biological, but that 
the differences between them are not altogether to be neglect- 
ed. The most important common feature which unites them 
and which causes them to be regarded as sub-formations and 
not as formations is first the general occurrence of the desert- 
trees, though under a somewhat different form; secondly that 
the soil is sand, which is saline only in the depressions so 
that the true halophytes play a comparatively minor part 
except in these places. 
The different forms of sand-desert are evidently histori- 
cally related in that the one must have originated from the 
other. What has been the course of development? It has 
been already pointed out that RappE (1899 p. 16) following 
OBRUTSCHEW regards the Desert-plains‘ the covered Sand-steppe” 
as the last stage in the metamorphosis of the sand; that the 
sand-hills, while being covered by vegetation, are gradually 
being levelled down through the agency of water, wind and 
