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 different 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 tcpics. 
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 Okologischen Pflanzengeographie, 97. 1896. 
