Egg + 
kept the sand at the old level. Mac Doucar (1908 pl. 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- 
eryptophyte 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 severe 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 
