228 



ELEMENTS OF GENERAL SCIENCE 



random. Each kind of plant needs for its growth certain 

 conditions that may be partly or entirely different from the 

 conditions demanded by other plants. For this reason any 

 place in which the soil, moisture, temperature, and other con- 

 ditions are favorable to one or a few kinds of plants is likely 

 to be an unfavorable place for certain other plants. Thus, the 

 hilltop which affords just the right conditions for the growth 



FIG. 111. A desert landscape 



The plants are not sufficiently numerous to entirely cover the soil. (Southern 

 Arizona.) Photograph by Dr. D. T. MacDougal 



of oak and 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 then* 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. 112). There are many kinds of plant 

 associations, and these constitute an important subject of study. 



