438 GENERAL BOTANY 



fruits, like the nuts of the hickory and oak, the devices are less 

 effective and serve for local rather than for wide seed distribu- 

 tion. The seeds of edible fruits, like berries, apples, and cherries, 

 are also very widely disseminated by birds and other animals, 

 which cast the seeds with their excrements in regions far re- 

 moved from the home of the mother plant. Almost innumerable 



FIG. 284. Invasion of a grass, Agropyron, into bare sand by groups, Mount 

 Garfield, Pikes Peak, Colorado 



After Clements 



examples might be added of other devices by which the mobile 

 seeds, fruits, and other reproductive parts of plants migrate and 

 invade new regions. 



The immediate effect of such migrations and invasions (Figs. 284 

 and 285) in the formation of new plant associations is most easily 

 observed where tracts of land occur which are devoid of vegeta- 

 tion. Such land surfaces may exist in gardens, lawns, and fields, 

 or they may be the result of fire, flood, or other destructive 



