PLANT ASSOCIATIONS 325 



In any given area the amount of available water may 

 not remain the same. For example, the margins of ponds 

 may slowly encroach upon the open water; ponds may be- 

 become converted into bogs; and bogs into dry ground. In 

 his drainage operations and removal of forests man has made 

 changes in the water-supply over extensive areas. All of 

 these changes involve the destruction of old plant associa- 

 tions and the coming in of new ones. 



188. Temperature. The temperature of the air and of 

 the soil during the growing season is very important in 

 determining the presence of different plants upon any area. 

 For each kind of plant there is what may be called a zero 

 temperature, below which it is not in the habit of work- 

 ing. The succession of plants during a single growing sea- 

 son illustrates the distribution of plants by temperature, 

 spring plants being able to endure greater cold than can 

 those of the summer. This distribution in time indicates 

 the more important distribution in space that is brought 

 about by differences in temperature. 



Permanent changes in the temperature of a region, af- 

 fecting the distribution of plant associations, are evident 

 only in tracing the history of plants back into what are 

 called geological times. At certain times arctic conditions 

 prevailed in regions now temperate, and this had an im- 

 mense influence on plant life. 



Plant associations are not determined by one condition, 

 but by a combination of conditions. The simplest illustra- 

 tion of this fact may be obtained by combining the water 

 and the temperature conditions. For example, if there is a 

 combination of scanty water with high temperature, a de- 

 sert is the result; but if the combination is abundant water 

 and high temperature, luxuriant vegetation is the result. 

 Since the possible combinations of water-supply, tempera- 

 ture, and other conditions are endless, it is evident that 

 there are very numerous plant associations. 



