CHAPTER XII 

 AGGREGATION AND MIGRATION 



251. Aggregation. The coming together of individual plants is 

 the process that produces vegetation. It gives rise to the innumer- 

 able groups of varying rank which taken together make up vegeta- 

 tion. This process is aggregation. In its simplest form aggregation 

 is the immediate result of reproduction, but as a rule the movement 

 or migration of the individuals plays an equally important part. 

 The degree and kind of aggregation are consequently determined 

 by the relation between reproduction and migration. Aggregation 

 is also affected by the abundance of the parent individuals, which 

 may occur singly or in groups. 



The simplest cases of aggregation are independent of migration. 

 Aggregation gives rise at once to competition between the aggre- 

 gated individuals, and upon the outcome of this depends adjust- 

 ment or establishment. In the majority of cases simple aggrega- 

 tion is prevented by the migration of the seeds or fruits away 

 from parent plants. The result, however, is the same, the final 

 grouping of the individuals depending upon competition and 

 ecesis. 



252. Simple aggregation. The simplest examples of this process 

 occur in such algie as Gloeocapsa and Tetraspora, in which the 

 plants resulting from fission are held together by a mucilaginous 

 substance. The relation between the plants is essentially that of 

 parent and offspring, even when the parent disappears regularly 

 as in the fission algaj, or sooner or later as in the case of annuals. 

 Such a group of individuals is a family, and corresponds more or 

 less closely to the family in human society. Practically the same 

 grouping occurs in the case of terrestrial forms, flowering plants 

 especially, when the seeds of a plant mature and fall to the ground 

 about it. The size and density of the family group are dctorminod 



