CAUSES 11 



new plants is the rule. In very many cases, however, this 

 primitive tendency is largely or completely negatived by the 

 presence of special dissemination contrivances, which are 

 nearly, if not quite, as effective for many terrestrial plants, 

 as the free floating habit is for algae. From this point, the 

 whole question of mobility belongs to migration, just as the 

 adjustment between the parent plants and their offspring, 

 or between plants established and the mobile plants to be 

 established belongs to occupabion. 



If association were determined by reproduction and im- 

 mobility alone, it would exhibit areas dissimilar in the mass 

 of individuals, as well as areas dissimilar in the kinds of in- 

 dividuals. Some areas would be occupied by plants of a 

 single species, others by plants of several or many species. 

 This tendency of association to show differences is, however, 

 greatly emphasized by the fact that vegetation is funda- 

 mentally attached to and dependent upon a surface that ex- 

 hibits the most extreme physical differences. For this 

 reason, new differences in association appear, due not only 

 to the morphological differentiation of vegetation forms, but 

 also to the changes in the degree and manner of association 

 produced directly by the different habitats. Association 

 might then be defined as a grouping together of plant in- 

 dividuals, of parents and progeny, which is initiated by re- 

 production and immobility, and determined by environment. 

 It is a resultant of differences and similarities. In conse- 

 quence, association in its largest expression, vegetation, is 

 essentially heterogeneous, while in those areas which possess 

 physical or biological definiteness, habitats and vegetation 

 centers, it is relatively homogeneous. This fundamental 

 peculiarity has given us the concept of the formation, an 

 area of vegetation, or a particular association, which is 

 homogeneous within itself, and at the same time essentially 

 different from contiguous areas, though falling into a phy- 

 logenetic series with some and a biological series with 

 others. From its nature, the plant formation is to be con- 

 sidered the logical unit of vegetation, though it is not, of 

 course, the simplest example of association. 



