. FORMS OF VEGETATION. 229 



night, from day to day, and from season to season. It is 

 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 completely 

 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 injuri- 

 ous 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 rainfall are likely to be associated. 

 It is possible, in a somewhat arbitrary way, to recognize four 

 general sets of conditions to which plants must adapt them- 

 selves, in each of which the relation to water is the dominant 

 factor. It should be understood clearly, however, that these 

 sets of conditions pass into each other imperceptibly. Cor- 

 responding 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 possi- 

 ble to distinguish four forms of vegetation corresponding to 

 the four sets of external conditions. 



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

 acterized by no extremes. Both the air and the soil are mod- 

 erately moist; the rainfall is distributed through the year, or 

 at least through the growing season ; there is no excess 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 



