83 



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 PALEZKIJ, 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. PALEZKIJ 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* 



