220 



PLANT PHYSIOLOGY AND ECOLOGY 



the activities of groups of plants are aggregation, migration, estab- 

 lishment, reaction, and competition. Migration and ecesis con- 

 stitute the basis of invasion, while reaction and competition are 

 typical of the complete invasion which causes one formation to 

 replace another, i.e., succession. 



In conjunction with the habitat, these functions produce and 

 modify the grouping of the individuals and species of the forma- 



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.::y.7 . ,.; 



1QiR 



i^m 



/ JE ES?IX!iS'^ 



Fig. 8L A willow thicket formation, consisting of several species of Salix, 

 just above timber line on Pike's Peak. 



tion. As has already been seen in the case of the plant, the activi- 

 ties of the formation furnish the clue to its structures. The latter 

 are more or less definite areas produced by variations in the habitat, 

 by the activities of the formation, or, in the majority of cases, by 

 both. In addition formations are themselves grouped together 

 in such a way that vegetation itself shows a more or less definite 

 structure. In both cases, two distinct types of arrangement or 

 structure may be distinguished. In the one the parts of forma- 

 tions or the formations themselves are arranged in zones, and the 

 type of structure is called zonation. In the other the areas show 



