HE LET | ee 
form, is strengthened by the fact that the species in their 
outer and inner structure agree most closely with alpine 
plants, in other words they are more strongly influenced by 
the altitude above sea-level, than by the dryness of the 
climate. 
CHAPTER 7 
The Vegetation of the Mountain-Slopes. 
(Eurotia-formation, Arenaria Meyeri-formation, and 
i Poa attenuta-formation.) 
In writing of the vegetation (Trigonella-formation) of hori- 
zontal flats, attention was called to the fact that other plant 
species than those of the flats are to be found in depressions, 
even though these are quite small and shallow. Arenaria 
Meyeri is characteristic of such depressions and is only found 
there (on flats). An example was given of the vegetation on 
a flat, mainly horizontal, yet slightly billowed, and we found 
that the species change according to the different exposures, 
even though the angle of declivity is extremely small. (Above, 
page 66.) Thus the adaptability of the plants on flats to the 
conditions of moisture or of exposure is very great. Expo- 
sure should presumably merely be considered as another 
expression for conditions of moisture, in that it determines 
the angle at which the sun’s rays strike the surface of the 
ground during the different hours of the day, and the length 
of the period during which the locality is sunny. 
Climbing a mountain in Pamir and’ going down on the 
other side, very soon reveals to one the enormous importance 
of exposure, and one quickly realizes that exposure is, in 
general, the determining factor for plant growth on a moun- 
tain slope. Roughly speaking, a northern and eastern exposure 
are favourable, a southern and western unfavourable for the 
appearance of a luxuriant vegetation rich in species; plants 
on a southern and western exposure are xerophytic, while 
those on an eastern, and especially a northern exposure are 
more or less mesophytic. 
