— 97 - 



kept the sand at the old level. Mac Dougal (1908 pi. 2) gives 

 a picture of a sand-hill formed in the same way by a species 

 of Rhus. 



All the herbaceous plants given for the shifting desert 

 occur in the Hummock-desert, where they live under more 

 favourable conditions, because less exposed to burial by the 

 sand or to exposure by denudation. Amongst other herbs 

 occurring in the Hummock-dessert the most important is 

 Carex physodes which, though mainly a spring-plant, yet plays 

 a considerable part throughout the summer. It is a hemi- 

 cryptophyte with sympodial, horizontal rhizomes, which to- 

 gether with the branched roots form a network in the sur- 

 face-soil (fig. 15). The growth is so dense that during spring 

 Carex physodes forms a green-sward in places. In June the 

 leaves have already withered, and the resting summer-buds 

 are hidden in a tunic of dead leaf-sheaths. The plant plays a 

 prominent part in binding the sand, but it cannot contend 

 against a sevei-e sand-drift. This plant and Aristida pennata 

 do not thrive together because the latter is only luxuriant in 

 shifting sand, Carex where it is stable. 



Alhagi Camelorum is very common in many parts of the 

 stable desert. It spreads vegetatively by aerial shoots produced 

 from long, horizontal roots. The part above ground is annual, 

 poorly provided with leaves, thorny and often globular. It is 

 very hardy, and when buried it forms new aerial shoots from 

 the leaf-axils of the old shoot, while if the sand is blown 

 away, new aerial shoots arise from the subterranean parts. 

 Alhagi may occur as a plant of the dunes under apparently 

 unfavourable conditions, but it seems to depend on tbe 

 ground-water not lying too deep. Like many other plants in 

 the neighbourhood of oases, it is used for fuel. 



Other plants in the Hummock-desert are: Tournefortia 

 sibirica, similar in habit to a Lithospermum, rather strongly 

 hairy, with white blossoms and light fruits which the wind 

 gathers together in sheltered places; Convolvulus divaricatus 

 is woolly-haired with small cordate leaves; Pluchea caspica 

 and Jurinea derderioides are thin-leaved knap-weeds; Goebelia 

 pachycarpa is one of the Papilionaceae with pinnate hairy 

 leaves; Haplophyllum obtusifolium is a bright green glabrous 



7 



