64 BASES AND CRITERIA. 



climax. In the development of a sere, extreme conditions as to water yield 

 to those more and more favorable to growth, and this change is accompanied 

 by a sequence of dominants belonging to successively higher vegetation-forms. 

 In short, the more striking indicator values of succession are afforded by the 

 changes from one vegetation-form to another, just as those next in importance 

 are marked by the change from one associes to another of the same form. 

 Moreover, while the exact significance of any species can be known only by 

 determining its functional response to the factors of its habitat, its general 

 meaning is indicated by the vegetation-form to which it belongs. 



Raunkiaer (1905, 1908; Smith, 1913: 16) has employed his system of vege- 

 tation-forms to determine the climatic relations of a particular flora. He 

 establishes a hypothetical normal spectrum for the whole earth by selecting 

 1,000 representative species, of which 400 were carefully analyzed. The bio- 

 logical or phyto-climatic spectrum of a particular region is obtained by finding 

 the percentage of species belonging to each life-form. Raunkiaer's method 

 adds interest and detail to the long-accepted relations between climate ahd 

 flora. It can not be applied to vegetation and hence it has no real indicator 

 value, as is shown by the author's own statements (1905 : 433) : 



"If we consider the flora of Denmark, it is characterized from the botano- 

 climatic viewpoint by its hemicryptophytes and not by its phanerophytes, for, 

 however important may be the role played by the forests in the vegetation of 

 Denmark, the small number of species of phanerophytes is significant of the 

 conditions offered by this region: The species of phanerophytes represent but 

 6 to 7 per cent of those living in Denmark, while the henriicryptophytes con- 

 stitute nearly a half of all the species. 



"But from the standpoint of the formation, the phanerophytes, or trees, 

 dominate by their size wherever one finds them. In spite of the inferiority in 

 number of the species of phanerophytes to those of hemicryptophytes or 

 crj-ptophytes, our forests belong to the phanerophytic formations because the 

 phanerophytes they contain dominate the other components of the forests." 



HABITAT-FORMS. 



Concept and history. In addition to the taxonomic form and vegetation- 

 form, species exhibit a form which is much more distinctly related to the 

 habitat. These usually bear the clear impress of the latter and hence are 

 called habitat-forms. The fuller recognition of their basic importance by 

 Warming (1895, 1896 : 116) was largely responsible for the rapid development 

 of ecology during the last two decades. Unlike taxonomic forms and vegeta- 

 tion-forms, their value is primarily ecological and not floristic, and they are of 

 correspondingly greater importance as indicators. Their significance lies in the 

 fact that they bear the primary impress of the controlling or limiting factor, 

 and thus serve as direct indicators of the critical factors of the habitat. They 

 are the essential basis of all indicator values, and must be regarded as the 

 main objective in all such studies. 



Warming's system. Warming (1896 : 116) was the first to adequately 

 organize the four universally known groups of habitat-forms, namely, hydro- 

 phytes, xerophytes, halophytes, and mesophytes (cf. Clements, 1904 : 20). 

 Pound and Clements (1898: 94; 1900 : 169), feeling the need of recognizing 

 light as well as water, divided mesophytes primarily upon the basis of light 



