328 FOUNDATIONS OF BOTANY 
for not less than nine months of the year. The flowers 
are often showy and appear very quickly after the brief 
summer begins. Mosses and lichens are abundant, — the 
latter of economical importance because they furnish a 
considerable part of the food of reindeer. 
400. Mountain or Alpine Vegetation. — In a general way 
the effect of ascending a mountain, so far as vegetation is 
Fig. 232.— A Plant of Arctic Willow. (About natural size.) 
concerned, is like that of traveling into colder regions. 
It was long ago suggested, in regard to Mount Ararat, 
that on ascending it one traversed first an Armenian, then 
a South European, then a French, then a Scandinavian, 
and finally an arctic flora. Up to a certain height, which 
varies in different latitudes, the slopes of mountains are 
very commonly forest-covered. The altitude up to which 
trees can grow (or as it is commonly called in this country 
the “ timber line”) is somewhat over twelve thousand feet 
