274 PRACTICAL COURSE IN BOTANY 
still and sluggish, or in rapid motion. It may exist more 
or less permanently in the atmosphere, as in moist climates 
like those of England and Ireland, where vegetation is 
characterized by great verdure; or it may come irregularly 
in the form of sudden floods, or at fixed intervals, causing 
an alternation of wet and dry seasons. Moreover, the 
moisture of the soil or the atmosphere may be impregnated 
qi pine : 
a a Ge es a 
Fie. 410. — The effect of cold —a Mt. Katahdin bog. (From Mo. Botanical 
Garden Rep’t.) 
with minerals or gases, which may affect the vegetation 
independently of the actual amount of water absorbed. 
Snow is a form of water which may act in two entirely 
opposite ways: (1) by keeping the atmospheric precipita- 
tion locked up in a solid state and thus bringing about a 
condition analogous to drought —for example, in arctic des- 
erts and Alpine snow fields; (2) by causing annual floods 
and overflows when it melts in the spring, as in the Nile 
and Mississippi valleys. 
In cold temperate regions it also influences vegetation 
