PLANT ASSOCIATIONS 825 
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. 
