196 SOME BRITISH PLANT GROUPS 
As we mount higher and higher on the hills, plants 
become fewer and more stunted, but hardy forms 
persist even long after the level of perpetual snow is 
reached. In the Alps, Ranunculus glacialis occurs up 
to an elevation of about 14,000 feet. In West Tibet, 
strange stunted species of Saussurea, a genus of 
Composite allied to the Thistles, exist at elevations of 
17,000 to 19,000 feet. Some of the Cryptogams go 
higher still, Lichens grow on the summit of 
Kilimanjaro (over 19,600 feet); and Schimper sug- 
gests* that this may by no means represent the 
absolute limit of vegetation. The prevalence of snow 
and ice does not of itself inhibit the lower forms of 
life. Since “red snow” was shown, nearly a century 
ago, to be due to colonies of a minute Alga, many 
microscopic organisms of like habitat have been dis- 
covered, and these algal colonists of snow and ice 
are now known to extend far over the frozen deserts 
of the highest hills, and to penetrate into the remotest 
regions of the Arctic and Antarctic. © 
As we get up to the level of perpetual snow on the 
higher mountains, or go northward within the Arctic 
Circle, the conditions under which plant life exists 
become very severe. It has been pointed out that in 
spite of a superficial similarity, wide disparity exists 
between the sets of conditions prevailing in the two 
kinds of habitat just mentioned. In the Arctic the 
winter is continuously dark and the summer continu- 
ously light; and in summer the sun is never far above 
the horizon, so that the temperature remains low, 
though it rises amply far enough above freezing-point 
* A. F. W. ScHiIMPER: ‘‘ Plant Geography ’’ (English transla- 
tion, 1903), p. 719. 
