170 THE HIGH ALPINE PLANTS 



difficulties. On the other hand, the extreme short- 

 ness of the flowering season at high altitudes is 

 probably the factor which, more than any other, hmits 

 the upward distribution of Alpines. Time for 

 flowering alone is not sufficient. A certain length 

 of time afterwards must also be available to set 

 and distribute the seed, to enable the ofispring to 

 take a firm hold in its new home, and to allow the 

 parent to manufacture reserves against the coming 

 winter. 



The so-called snow-line, or imaginary line above 

 which the snow continues to lie all through the 

 summer, varies in elevation according to the aspect, 

 situation, and other physical conditions of any parti- 

 cular locality. In general, it lies between 8,500 and 

 10,500 feet, but is sometimes lower or even higher. 

 Though above this elevation the coating of snow is 

 permanent, it is by no means continuous. Rocks 

 fully exposed to the sun, steep slopes, and precipitous 

 crags quickly lose their snowy covering, for a time 

 at least, in summer, though the periods during 

 which they are free from snow may be only short and 

 intermittent. 



But bare rock alone will rarely furnish a liveli- 

 hood sufficient to permit the seed of a High Alpine 

 to establish itself. Some sort of primitive soil, such 

 as those which we have already discussed (p. 96), 

 must in most cases be present. In the High Alpine 

 region the thalli of Crustaceous Lichens appear to 

 contribute in a large degree to the building up of 



