ASSOCIATION 



203 



252. Aggregation. As indicated under the causes of association, the 

 process by which groups of individuals are formed depends entirely upon 

 reproduction and migration. In short, aggregation is merely a corollary of 

 movement. The simplest example of this process occurs in forms like Gloe- 

 ocapsa, Tetraspora, and others, where the plants resulting from fission are 

 held together by means of a sheath. Though called a colony, such a group 

 of individuals is a family in the ordinary sense. Practically the same group- 

 ing resulis in the case of terrestrial plants, especially spermatophytes, when 

 the seeds of a plant mature and fall to the ground about it. The relation in 

 both instances is essentially that of parent and offspring, although the parent 

 soon disappears in the case of annuals, while among the algae its existence 

 is regularly terminated by fission. The size and the density of the family 

 group are determined by the number of seeds produced, and by their mo- 

 bility. These are further affected by the height and branching of the plant, 

 and by the position of the seeds upon it. The disseminules of immobile 

 species fall directly, beneath the parent, and the resulting group is both uni- 

 form and definite. A similar arrangement is caused likewise by offshoots. 

 An increase in mobility brings about a decrease of aggregation, since the 

 disseminules are carried away from the parent plant. Perfectly mobile 

 forms rarely produce family groups for this reason. It is evident, however, 

 that mobile perennials sometimes arrange themselves in similar fashion in 

 consequence of propagation by underground parts. Consequently, it is pos- 

 sible to state the law of single aggregation, viz., that immobility promotes 

 the grouping of parent and offspring, and mobility hinders it. 



If all species were immobile, the family group would be characteristic of 

 vegetation. Since the great majority are more or less mobile, aggregates 

 of this sort are the exception rather than the rule. Alobility not only de- 

 creases the number of offspring in the family group, but it also spreads dis- 

 seminules broadcast to enter dissimilar groups. It leads directly to mixed 

 aggregation, by which individuals of one or more species invade the family 

 group. Once established, the newcomers tend also to produce simple groups, 

 thus causing an arrangement corresponding essentially to a community. 

 Such collections of family groups are extremely variable in size and defini- 

 tion. This arises in part from the nature of simple aggregation, and in part 

 from the varying mobility of different species. Mobility alone often pro- 

 duces similar communities by bringing together the disseminules of different 

 plants, each of which then becomes the center of a mixed group. In the 

 case of permobile species, several disseminules of each may be brought to- 

 gether. The resulting area, though larger, is practically the same. At pres- 

 ent, it is difficult to formulate the law for this method of grouping. It may 

 be stated provisionally as follows : mixed aggregation is the direct result of 

 mobility, and the greater the mobility the more heterogeneous the mixture. 



