164 AGGREGATION, COMPETITION AND CYCLES 



paramount, in accordance with the general rule that the factor pres- 

 ent in the smallest amount relative to the demand will be the critical 

 one. 



"With respect to layered communities, water is the decisive factor 

 in the competition of the dominants of forest, scrub, or grassland. 

 The grouping into the lower layers is largely an outcome of the com- 

 petition for light, but within each layer it is probable that water is 

 more important. However, in all but the most open stands, both water 

 and light are concerned, and their relative significance is to be deter- 

 mined only by a specific study. 



The Equipment of Plant Competitors. The most significant fea- 

 ture of competing plants is the life form, since this determines the 

 behavior and relations of shoot and root. It bears the ecological 

 impress of climate and soil, and hence largely determines the response 

 to the direct physical factors, as well as to the reaction due to compe- 

 tition. The most telling characteristic of the life form is the duration 

 or length of the life period, as a result of its effect upon occupation 

 and to a large degree upon stature as well. Next in importance 

 comes the rate of growth, which finds its most effective expression in 

 the expanse and density of shoot and root systems, and the depth of 

 the latter. Rate and amount of germination often yield an initial 

 advantage difficult to overcome later, and these are related to the 

 number and kind of seeds produced, which in turn are influenced by 

 the competition among flowers for pollinators. Vigor and hardiness 

 of root or shoot may also play a decisive role, since they frequently 

 determine survival under unfavorable conditions, and especially 

 under the stress of winter. Such qualities may be inherent in the proto- 

 plasm itself, but as a rule they are related to growth and structure, 

 and particularly to ripening and dormancy. 



In the final outcome, the species with the best equipment will 

 furnish the dominants for the plant matrix, while those with some- 

 what less advantageous features will become subdominants. It is 

 doubtless a matter of primary significance that the dominants of 

 climaxes are drawn almost wholly from the three types of life form, 

 namely, trees, shrubs, and grasses. 



Competition among Flowers. In contrast to the competition among 

 plants which rests chiefly upon the life form, that between flowers is 

 based in some measure upon the taxonomic form. The actual success 

 in attracting pollinators in large number depends mostly upon the 

 size, color and odor of the flower, but with respect to effective visitors 

 the form of the corolla and the arrangement of stamen, stigma, and 



