THE PLANT COVERING OF THE EARTH 317 



partly or entirely different from the conditions demanded by 

 other plants. For this reason any place in which the soil, 

 moisture, temperature, and other conditions are favorable to 

 one or a few kinds of plants is likely to be an unfavor- 

 able place for certain other plants. Thus the hilltop, which 

 affords just the right conditions for the growth of oak and 



FIG. 162. Plant associations on the margin of a pond 



Note the rushes in the water, the low-shrub and tall-shrub associations beyond, 

 and the tree association in the background 



hickory trees, is not equally favorable to the growth of elm 

 or willow, and these are not usually found there. Plants 

 which can find suitable conditions for their growth at the 

 same place are likely to be found growing together. A group 

 of plants which are commonly associated in this way is 

 known as a plant association (fig. 152). There are many 

 kinds of plant associations, and these constitute an impor- 

 tant subject of study. Those types of plant associations in 

 which trees form the most conspicuous part are known as 



