AGGREGATION 57 



cant of these are reaction, competition, coaction, and aggregation 

 and ecesis (succession) , and they are in consequence discussed in 

 considerable detail in the following chapters. For the others the 

 general account given here is supplemented by discussion or specific 

 mention of their role in the biome later in the text. 



AGGREGATION 



As a technical terra, aggregation was first employed in ecology 

 in the dynamic sense, for the coming together of individuals as a 

 result of multiplication (Clements, 1905, 1928). With respect to ani- 

 mals, it has occasionally been given a similar meaning, but more 

 recently Allee (1931, a) has used it as the inclusive term for the 

 groups that result from the process. In this sense, it is largely 

 synonymous with community, though more exactly with the family 

 or colony of plant ecologists. However, no serious ambiguity is in- 

 volved in the double use of the word, which will probably continue 

 until the study of minor communities becomes much more general on 

 a developmental basis. 



Aggregation among Plants. In its simplest form, aggregation is 

 the direct consequence of multiplication, though as a rule it is also 

 dependent upon migration in some degree. The first type is exem- 

 plified by such algae as Gloeocapsa and Nostoc, in w^iich the dividing 

 cells are held together by a matrix of mucilage. Such a group of 

 individuals is a family, the relation being essentially that of parent 

 and offspring, even though the parent disappears as a result of fission. 

 Practically the same type of grouping occurs in terrestrial forms, 

 especially flowering plants, when the germules mature and fall to the 

 ground about the parent. A family is also produced when propagation 

 by offshoots leads to a similar disposition. All these are instances of 

 simple or primary aggregation, in which migration is absent or slight. 

 This is often the case in annuals wdth high seed production, and, in 

 consequence, these supply by far the largest number of pure families. 

 Moreover, the conditions for simple aggregation are especially favor- 

 able in bare areas and secondary ones in particular, so that the initial 

 stages of subseres are regularly characterized by annuals (Fig. 12). 



]\Iixed or secondary aggregation ensues w^hen seedlings (germules) 

 of two or more species become intermingled to form a colony, in the 

 case of a bare area, or when migration carries propagule or seed into 

 an established family. Every community is an example of this type 

 of grouping to some degree, in view of the fact that association is 

 merely the outcome of the interplay of aggregation and migration. 



