RATS NE 
continue to grow and to struggle against the sand after the 
older main shoots have flowered. Horizontal runners are not 
formed, so far as I know. According to PALEZKW, Aristida 
forms two kinds of roots, long horizontal anchoring roots, 
and shorter perpendicular absorbing roots. The roots are 
protected by a “sand-stocking” or cover in which the other 
root-tissues are loosely enclosed. The leaves are able to roll 
up like those of Psamma, and have green tissue on the pro- 
tected upper surface. 
In the shifting sand-desert, all other vegetation is depend- 
ent on the presence of Aristida pennata. 
This plant is not merely a sand-binder, but plays an 
even more important part because its dense tufts are practi- 
cally the only place, where seeds of other plants can secure 
a roothold. PaLezkiy has drawn attention to this fact, and I 
have frequently observed, that the fruits of the switch-bushes 
find a resting place in the Aristida tufts where they are re- 
tained amongst the leaves and shoots and are frequently 
covered by sand. These fruits easily roll about in the wind 
so that they would hardly ever germinate in the shifting 
desert if it were not for the tufts. In more stable parts, the 
conditions are more favourable and there is generally suffi- 
cient vegetation for the fruits to establish themselves. 
The plants which come next after Aristida, and perhaps some 
of the annuals, are the switch-bushes, a very characteristic type. 
In the sand-desert the most hardy is the Sand Acacia (Am- 
modendron Conollyi (fig. 10) and Karelini). It occurs as slender 
trees or low shrubs standing hundreds of metres apart. Grey 
in colour, it has small narrow leaves thickly coated with 
silky hairs, and light passes through the crown so that only 
a slight shade is cast (see Lipsky 1911 tab. 1). In the valleys 
between the barchans it may form a tree with a trunk and 
elegant hanging branches, but sometimes where the barchan 
has swept over the trees only the tops of the crowns are seen 
protruding above the sand. The strength of the Sand Acacia 
lies in its height, its long roots (19 metres, PALEzKIJ) and its 
small leaves. 
If this plant can hold its own and produce seeds from 
the butter-coloured one-seeded samaras which ripen in May, 
6 
