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seen. Its foliage is still whiter than that of the poplar, so 
that this forest is not at all like the northern bright-green forests. 
The poplar forest may be quite dense, but not hard to 
traverse except where the undergrowth is dense, and this 
only occurs where light penetrates the tree-canopy. The prin- 
cipal species of the undergrowth are: the thorny bush Hali- 
modendron argenteum, Erianthus Ravennae and Tamarix, these 
species, more especially Tamarix, cannot grow in deep shade. 
The other species present, Glycyrrhiza glabra, Aeluropus litto- 
ralis and Alhagi Camelorum, can endure shade, but it is very 
noticeable how much wider apart they stand here than in 
the light. When they form undergrowth they are so scattered 
that one may walk about without treading on them, where- 
as in the light they usually form a close mat. — Open glades 
in the forest where Halimodendron and Erianthus occur, seem 
in many cases to be due to human agency. Trees are cut 
down for building purposes and for fuel. 
In the Populus pruinosa forest, lianes are represented: 
Cynanchum acutum, Asparagus verticillatus, Clematis orientalis 
(f. oblonga). These may be seen climbing high into the trees, 
but neither so frequent nor so luxuriant as to allow it to be 
said that they give the forest a special character. This may, 
however, be said of Cuscuta Lehmanniana, the growth of which 
in many parts is so luxuriant that it kills large branches and 
even whole trees. 
Along the river-banks in the Chiwa country Populus euphra- 
tica sometimes forms forest. It has the same grey leaves as 
P. pruinosa, but their shape is still more variable (see fig. 20), 
the leaves of the bushes being linear while those of the 
trees are as broad as the broad leaves of P. pruinosa. P. eu- 
phratica attains about the same height as P. pruinosa. It may 
perhaps only be accidental, but where I have found it as the 
chief species in the thicket it was more ashrub than a tree 
with a trunk, and it seemed to be less inclined to form a 
close canopy than P. pruinosa. One result of this is that the 
P. euphratica association is less forest-like and has more the char- 
acter of a thicket. These thickets may be very dense, some- 
times they are almost impenetrable because of the thorns of 
Halimodendron argenteum. In addition to this species there 
