SOCIAL LIFE AMONG PLANTS 13 



with a therophyte aspect, which later is replaced by a bulb geophyte, 

 hemicryptophyte, or chamaephyte aspect, following each other in the 

 course of the year on the same limited area. 



That the competition between the species of a community is the 

 more intense the more closely these species agree in their requirements, 

 life forms, and seasonable development may be accepted as a general 

 principle. 



The more manifold the structural units of- a community are the 

 better will all the space be utilized and the greater, as Darwin has said, 

 will be the sum of life per unit area. 



Aggressiveness of Species. — The possibility of numerous species 

 living together in limited space is largely affected by the aggressiveness 

 of the dominant species. The aggressiveness of a species depends upon 

 its fecundity, its rapidity of reproduction and spread (formation of 

 clumps or masses), and its capacity to occupy its place permanently. 

 Carex curvula, an alpine tussock plant of a high degree of sociability, 

 hinders by its dense stands the invasion of companion plants into the 

 Curvuletum, In the nearly related Festucetum halleri, on the con- 

 trary, there are no dominant species of such overwhelming aggressive- 

 ness, and the density of stand, and the diversity of species of companion 

 plants on the same area, is much greater (cf. p. 55). 



Morphoecologic plasticity favors a plant in the struggle of life 

 (Polygonum amphibium, Hedera helix, Calluna, etc.). The cosmo- 

 politan Phragmites communis, which is ecologically (but not morpho- 

 logically) extremely plastic, thrives equally well in the brackish 

 marshes of the southern European coasts, in central Asia, in the United 

 States of America, and in South Africa, in the cold meadow moors of the 

 lower Alps, and under the continually humid skies of the Atlantic 

 coast of Europe. 



The aggressiveness of species must be empirically determined, but 

 the morphobiologic framework of the species — the life form — gives a 

 basis for an estimate. In the temperate zones the tree form is superior 

 to the shrub, and this to the chamaephyte. Annuals are here far 

 inferior to all other life forms; nevertheless in the arid regions of the 

 subtropics they dominate wide areas. The aggressiveness of a definite 

 life form is therefore connected with the climatic region. It is also 

 related to the type of habitat. Every climatic region and every type 

 of habitat favor certain classes of life forms. From these are recruited 

 the species most efficient in competition {cf. chapter on Life Forms). 



Determination of Aggressiveness. — Experimental investigations of 

 aggressiveness in pure cultures of one species and in mixed cultures of 

 several species, under controlled conditions of soil and climate, have 



