BASES AND METHODS OF DETERMINATION. 55 



INDICATOR CRITERIA. 

 Nature and kinds of criteria. — Every response of the plant or community 

 furnishes criteria for its use as an indicator. These are most serviceable when 

 they are visible, but demonstrable functional responses may be even more 

 valuable, though invisible. The evidence as to functional responses in natural 

 habitats is still very limited, and will be considered in the next chapter under 

 the factors concerned. Here the discussion is confined chiefly to the criteria 

 afforded by form and structure, with which growth is included. The develop- 

 ment of the community is also considered along with its structure for the same 

 obvious reasons 



Criteria may first be divided into two kinds in accordance with their relation 

 to the individual plant or to the plant community. Individual criteria are 

 phylogenetic when they have to do with species and genera, and ecological 

 when they relate to life-forms and habitat-forms. It is probable that these 

 are all ecological responses, and that species and genera are more remote in 

 origin and hence their ecologic significance less evident. Life-forms are less 

 remote and their dependence upon the habitat more evident, while habitat- 

 forms are mostly of more recent origin and their relation to the habitat obvi- 

 ous. This view seems to be supported by the fact that it has proved impossible 

 to make a system of life-forms which is not based in part upon taxonomic 

 forms and in part upon habitat-forms. All of these criteria permit still finer 

 analysis, as species into varieties and forms, and habitat-forms into those pro- 

 duced by local or minute habitats. The experimental study of species and 

 life-forms is still too slight for such a procedure, and it is possible as yet with 

 only a small number of habitat-forms. The consideration of indicator criteria 

 is based upon the following divisions: (1) species and genera; (2) life-forms; 

 (3) habitat-forms; (4) growth-forms; (5) communities. 



Species and genera. — Quite apart from the life-forms and habitat-forms 

 which they exhibit, species and genera, and to some extent families also, have 

 an indicator value dependent upon their systematic position. The latter is 

 determined primarily by the responses recorded in the reproductive structures 

 at a time relatively remote. Their indicator meaning is consequently often 

 obscure, and this obscurity is increased by a complete lack of experimental 

 knowledge as to the factors which originate reproductive characters. Thus, 

 while many species and genera show correlations with habitat or climate, this 

 is chiefly on the side of vegetative responses, such as the relation of the Nym- 

 phaeaceae to bodies of water. They often exhibit, however, a valuable indirect 

 correlation with climate due to origin and migration. This is the basis of 

 floristic studies such as those of Sendtner (1856), Drude (1890), and others, and 

 of the more exact floristic methods of Jaccard (1901-1914) and Raunkiaer 

 (1905-1916). The value of these must remain statistical and general until 

 they are related to successional movements and to measured physical factors. 



Species and genera acquire their chief significance by virtue of the ecological 

 values involved in phylogenetic relationship. This is obviously true of all 

 genera which are largely or wholly consistent as to life-form, and it holds to a 

 considerable degree for all others. Habitat, successional, and indicator values 

 are concerned in this, and the genus thus becomes a sign of a more or less 

 definite ecological complex of responses. This is likewise true of species in the 

 general sense employed by Linne and Gray. A genus consists of several to 



