36 COMMUNITY FUNCTIONS— DYNAMICS OF BIOTIC FORMATION 



PLANTS 



Relation to Life Form and Habitat. The correlation between life 

 form and life cycle is not merely fundamental and comprehensive, 

 but it is likewise reciprocal in a high degree. Not only is the life 

 cycle the dynamic expression of the life form, but, in turn, the round 

 of behavior has a more or less definite and recognizable effect upon 

 the forms assumed by the organism. The first relation is well-nigh 

 universal wlien the phyletic or taxonomic form and the vegetation or 

 biome form are identical, as is often true of invertebrates and crypto- 

 gams. It is little in evidence in flowering plants, except for families 

 with highly specialized shoots, such as the grasses and cacti. Habitat 

 form and growth or competition form bear little or no relation to 

 taxonomic position, with the exception of such rare instances as the 

 water lilies, in which vegetation and habitat form are identical. The 

 influence of form upon the life history is compelling in the phyletic 

 or biome form; it is much diminished in that of the habitat form and 

 disappears more or less completely with the competition form. Con- 

 versely, the effect of life history upon morphology is greater wdth the 

 more recent forms, and it is either absent or little visible in the fixed 

 types. 



Outline. The following account is designed to serve a twofold pur- 

 pose. In the first instance, it is intended to provide a concise guide 

 to the study of particular life histories in such detail as the biotic 

 approach may warrant. In the second, it is to furnish a basis for 

 the investigation of coaction as the essential bond in the biotic com- 

 munity, as revealed by the interplay of the life cycles of the respective 

 plants and animals. This is illustrated by frequent correspondence 

 between active periods in animal life histories and the flowering of 

 seasonal groups of plants which supply food or shelter. 



Number of Stages. It is obvious that all plants agree in exhibiting 

 the three cardinal points or stages of a life history, namely, birth, 

 development, and death, or loss of identity at least, as in the case of 

 fission in unicellular algae. Likewise, it is clear that morphological 

 specialization reflects developmental history, with the consequence 

 that stages and activities increase in number from lower to higher 

 forms as a rule. In plants the principle holds without exception, 

 though the alternate generations of mosses and ferns are naturally 

 much more visible than those of flowering plants. 



In the great majority of plants, the life history begins with the 

 germination of spore and seed, is continued through growth and prop- 

 agation, and terminates in reproduction. In the simplest one-celled 



