436 



GENERAL BOTANY 



of species composing them or in the proportion existing between 

 the constituent species of each minor association. In a similar 

 manner, the xerophytic associations include lesser desert, dune, 

 cliff, bog, and saline plant associations, while mesophytic plants 

 form distinct associations in the form of forests, meadows, prairies, 

 and cultivated fields. These large and small plant associations 



FIG. 282. A new mesophytic forest association (twenty-five years old) of honey 

 locust, white elm, and black walnut 



Photograph furnished hy the United States Forest Service 



form the units which comprise the vegetation of local areas and, 

 finally, of the entire land surface of the globe occupied by plants. 



Origin of new associations. In order to understand the impor- 

 tant phenomena connected with the origin and development of 

 plant associations it will be necessary to consider certain dynamic 

 aspects of plant life, including the migration of plants from one 

 locality to another, the invasion and occupation of new territory 

 by such migrants, and the replacing of one plant population by 

 another in new regions. 



It is evident that the migration of plants by means of mobile 

 seeds, fruits, or other reproductive parts must be a potent factor 



