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 

 characterizcKl 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 



liotaiiK al 



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 



