310 PLANT LIFE. 



exposed to light varying in intensity from day to night and 

 from day to day, and to light differing in direction from hour 

 to hour of each day. It is enveloped by fogs or mists, or is 

 pelted by rain, hail, sleet, or snow, and sometimes com- 

 pletely buried in ice or snow. 



A plant has little or no power to alter any of the agents 

 which act upon it, but it must be able to withstand the in- 

 jurious ones, or even to turn them to its advantage. It 

 would be difficult to conceive a more complex set of factors 

 to which adjustment must be effected; and the more since 

 these conditions are combined with each other in an infinite 

 variety of ways. Because the physical conditions vary in- 

 different parts of the earth's surface, the vegetation in each 

 region differs from that in others. 



In any particular locality certain conditions of water, soil, 

 air, temperature, light, and precipitation are likely to be as- 

 sociated. It is possible, in a somewhat arbitrary way, to 

 recognize four general sets of conditions to which plants must 

 adapt themselves, in each of which the water supply is the 

 predominant factor. It should be understood clearly, how- 

 ever, that these sets of conditions pass into each other im- 

 perceptibly. Corresponding to these four sets of external 

 conditions, we may recognize certain characteristics in plant 

 form and structure, which are likely to be associated, and it 

 thus becomes possible to distinguish four forms of vegetation 

 corresponding to the four sets of external conditions. 



419. The first set of conditions consists of those charac- 

 terized by no extremes. Both the air and the soil are moder- 

 ately moist ; the precipitation is distributed through the 

 year, or at least through the growing season ; there is no ex- 

 cess of salts in the water or in the soil ; the soil is usually 

 enriched with organic matter, often in considerable amount. 

 The plants which grow under these conditions are the ones 

 most familiar to people in the fertile regions of temperate 



