a 
We know from HEINRICHER that isolateral leaf-structure 
seems to be especially dependent on strong light, and according 
to BONNIER the characteristic features of alpine plants, — 
short, hairy stems, small, hairy and relatively thick leaves, 
well-developed palisade tissue and a large number of stomata 
— seem to be determined by strong light and dry air, as each 
of these factors alone can transform a lowland plant in such 
a way that it is able to assume these characteristics to a 
greater or lesser extent (A. LOTHELIER). Strong light has, 
though, a greater effect than dry air. 
We may then explain the structure of the plants of the 
Pamirs as directly or indirectly dependent on the strong light 
and the dry air. 
But even though the dryness of the air is one of the fac- 
tors influencing the structure of the plants, the latter does not 
necessarily express a xerophytic adaptation. We know that 
the summers in Pamir are dry, and the plants must naturally 
be adapted to drought as well as to other conditions, — other- 
wise they could not live, — but we find only a few expres- 
sions of this adaptation to drought. 
We can only point to the small size of the leaves and 
their almost unfailing hairiness as frequent xerophytic cha- 
racteristics. The latter is especially striking and important, 
for both in the Alps, and in arctic fell-fields, many glabrous 
species are to be found, (the few indigenous to the Pamirs 
are named above), so that we here presumably have the ex- 
pression of the growth-forms for the particularly dry climate 
of Pamir. Added to this is the structure of the evergreen 
species, which, as mentioned above, are of a different and 
far more xerophilous type than that of the deciduous plants. 
Aside from this the plants do not seem to be xerophytically 
constructed, but both in regard to hibernation, and to the 
structure of the shoots and to the anatomy of the leaves, to 
agree in the main with alpine fell-field plants. The arctic 
species have as a rule, as shown by BÔRGESEN and by Bon- 
NIER, weakly developed palisade tissue in the leaves, and in 
this respect differ from the alpine species. 
The supposition emphasized above that the Trigonella- 
formation is rather to be considered a fell-field than a desert 
