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THE FORMATION 



vegetation form, in so far as this is governed by light. In those plants in 

 which the leaves are usually erect, notably the grasses and sedges, the com- 

 petition between the aerial parts is relatively slight, and the result is de- 

 termined by the reactions of the underground stems and roots. 



The position of the competing individuals is of the greatest importance. 

 The distance between the plants affects directly the degree of competition, 

 while their arrangement, whether in groups according to species or singly, 

 exerts a marked influence by determining that the contest shall be between 

 liWe forms, or unlike forms. Position is controlled primarily by the relation 

 existing between seed-production and dissemination. It is of course in- 

 fluenced in large measure by the initial position taken by the invaders into a 

 nudate area, but this is itself a result of the same phenomena. The in- 

 dividuals of species with great seed-production and little or no mobility 

 usually occur in dense stands. In these, the competition is fierce, for the 

 two reasons of similarity and density, and the result is that the plants fall 

 far below the normal in height and width. This is an extreme example of 

 the group arrangement. When the seed-production is small, the mobility 

 may be great or little without seriously affecting the result. The individuals 

 of a species of this kind will be scattered among those of other species, and 

 the closeness of competition will depend largely upon the similarity existing 

 between the two. The arrangement in such cases is sparse. A species with 

 great seed-production and great mobility usually shows both kinds of ar- 

 rangement, the position of the individuals and the competition between them 

 varying accordingly. This is due to the intermittent action of distributing 

 agents, making it possible for the seeds to fall directly to the ground during 

 the times that winds, etc., are absent. The three types of arrangement indi- 

 cated above are termed gregarious, copious, and gregario-copious. They 

 furnish the basis for the investigation of abundance which deals essentially 

 with the number and arrangement of the individuals of competing species. 

 The effect of distance, i. e., the interval between individuals, upon competi- 

 tion is fundamental. The competition increases as the interval diminishes, 

 and the reverse. 



The view here advanced, i. e., that competition is purely physical in nature, 

 renders untenable the current conceptions of vegetation pressure, occupation, 

 etc. Masses of vegetation are thought to force the weaker species toward 

 the edge, thus initiating an outward or forward pressure. As has been 

 shown above, no such phenomenon occurs in vegetation. This movement 

 is nothing but simple migration, followed by ecesis, and has no connection 

 with "weaker" species, or the development of a vital pressure. The direc- 

 tion taken by the migrating disscminulcs is essentially indetcmiinate. Mi- 

 gration seems to be outward, or away from the mass, merely because the 



