146 AGGREGATION, COMPETITION AND CYCLES 



rests in a large measure upon the cooperation of invading individuals 

 or species, while the progression of serai stages is the outcome of the 

 interplay between invaders and occupants in terms of cooperation and 

 competition. Finally, the problem of populations and their fluctuation, 

 of cycles, in short, is essentially a matter of aggregation, first of a 

 particular species in a mounting phase of numbers, and then of its 

 coactors, both predator and parasite, ushering in a second phase of 

 sharp decline in the entire complex. 



AGGREGATION AS A PROCESS 



Causes of Aggregation. The basic explanation of aggregation is to 

 be found in growth with consequent multiplication of individuals and 

 their grouping about the parents for a longer or shorter period. The 

 individuals once produced, the course of aggregation will depend upon 

 several considerations, such as the medium, the type of organism, 

 whether motile, mobile, or fixed, and the terrain, whether bare or occu- 

 pied. The process is at its simplest in forms that remain together 

 after fission, such as Merismopcdia and Nostoc among the algae, 

 Vorticella and Fuligo among protozoans, Volvox, and Hydra. Even 

 among flowering plants, the behavior is very similar when the seeds 

 or bulbils germinate on the parent plant, as in some onions. Polygonum 

 vivijKirum, etc. However, the almost universal rule among rooted 

 plants is for the spores or seeds to fall about the parent and give rise 

 to a family in bare areas or a colony in those already occupied. In 

 annuals, the family consists of one generation; in biennials and peren- 

 nials, of two or more. Moreover, aggregation in the latter is promoted 

 by offshoots of various sorts and may come to depend upon them 

 almost wholly, especially in cultivation. 



In the simple aggregation due to the fall of spores or seeds, gravity 

 takes the primary role, and it has also some part to play when other 

 factors enter. When transport occurs, simple aggregation is hindered 

 and mixed aggregation is favored, in more or less correspondence with 

 the efficiency of the migration device. However, even in the process 

 of migration, simple aggregation recurs frequently when obstacles are 

 interposed to the movement of wind, water, or soil, exemplified espe- 

 cially by windrows of tumbleweeds and wave lines of hydrophytes. 

 On a smaller scale, a similar result ensues when barbed fruits are 

 carried by animals or a rodent hoard is overlooked, though competition 

 between the seedlings usually prevents ecesis in such instances. In 

 the case of free algae, as well as of zoospores, aggregation may result 

 from the combined or separate action of wind, wave, or current, the 



