204 THE FORMATION 



The constitution of all the major areas of a formation is to be explained 

 upon the basis of aggregation by the two methods described. The relative 

 importance of family groups and communities differs for every formation, 

 and the exact procedure in each can be obtained only by the detailed study 

 of quadrats. The problem is further complicated by competition and reac- 

 tion, particularly in closed vegetation. For this reason, aggregation can be 

 studied most satisfactorily in a new or denuded area, where these processes 

 are not yet in evidence. 



Kinds of Association 



253. Categories. In the analysis of association, it must be kept clearly in 

 mind that the concrete examples from which all generalizations must be 

 drawn are often in very different stages of development, and are of corre- 

 spondingly difl'erent ages. For this reason it has seemed best to consider 

 the primary relations of association in general in this place, leaving the 

 treatment of the effects of invasion, succession, alternation, and zonation to 

 be taken up under these topics. 



Various categories of association may be distinguished, according to the 

 dominant physical factor concerned or the point of view taken. These will 

 fall into two series, as we consider the relation of plant to plant with refer- 

 ence to some object or characteristic, or the grouping of plants together in 

 response to some dominant factor. In the first series may be placed asso- 

 ciation with reference to substratum, to the ground (occupation), and to 

 invasion ; in the second belong light and water-content association. It 

 should be noted that these are all actual associations in nature, and not con- 

 cepts such as the vegetation form, within which plants from widely different 

 associations may be classified. Naturally, it does not follow that it is not 

 logical or valuable to group together those plants, such as hydrophytes, 

 sciophytes, hysterophytes, etc., which have a common relation to some factor, 

 but belong to different formations. 



254. Stratum association. Plants manifest independent or dependent as- 

 sociation with reference to the stratum to which they are attached and from 

 which they derive food or support. Independent association is exhibited by 

 those holophytic species of a formation which are entirely independent of 

 each other with respect to mechanical support or nutrition. It is charac- 

 teristic of the greater number of the constituent species of formations. De- 

 pendent association is manifested in the relation between host and parasite, 

 stratum and epiphyte, support and liane. Warming* has distinguished six 



^Lehrbuch der Okologischcn Pflanzengeographic, 97. 1890. 



