446 A TEXTBOOK OF GENERAL BOTANY 
The growing season is very short, about two months, so that 
the spermatophytes rapidly pass through the stages from flower- 
ing to fruiting. One of the most striking characteristics of the 
tundra is the abundance and large size of the flowers as con- 
trasted with the small, short stems. 
Mountain vegetation. The vegetation at different altitudes 
on mountains is profoundly affected by the changes in climate 

Fig. 501. Alpine meadow near Mount Robson, British Columbia 
at different elevations. With rising altitudes the temperature 
steadily decreases until, if the mountain is sufficiently high, 
there is a perpetual cap of snow. Rainfall is usually heavier 
on the lower slopes of mountains than in the surrounding low- 
lands. This is due to the fact that as the warm air from the 
lowland is forced up the mountainside it cools, with the result 
that its water-holding capacity is lessened, and the consequent 
excess of water in the atmosphere forms clouds, which frequently 
give rise to rain. The amount of rainfall increases up to a cer- 
tain altitude, and then decreases because, as the air continues — 
