CHAP, xxxv PHYSIOGNOMY OF VEGETATION 139 



As the number of associated species increases the number of growth- 

 forms as a rule does so likewise ; in this respect the premier place is 

 taken by the warm, moist tropical forest, which perhaps owes its boundless 

 wealth to the circumstance that it has been able to develop for vast 

 periods of time without interruption. 



The number of species depends, 1 inter alia, upon the means of competi- 

 tion possessed by the several species. Some species readily form dense 

 masses of vegetation composed of many individuals ; others are univers- 

 ally represented by isolated individuals. Many species can occur in several 

 kinds of formations, because the demands they make are bounded by 

 wide limits, and because the more habitats they can occupy the wider 

 are these limits. The hardiest and most accommodating species can 

 seize upon most kinds of habitat, nevertheless they are often found 

 only in a few, because they have been crowded out of the better ones. 

 The more peculiar and extreme a habitat is the more uniform does its 

 vegetation tend to be, because, as a rule, only few species are so specialized 

 in their adaptation as to be capable of existing in such a place. 



In studying the vegetation of a certain area from a floristic and geographical 

 standpoint, it is necessary to define the relative numbers of the various species. 

 Every community consists of dominant and sub-dominant species, as well as of others 

 that are more or less dependent upon these and occur only here and there. Drude 

 employs the following terms * : 



social : dominant species whose individuals give the main character to the 

 vegetation ; 



gregarious : species whose individuals occur in small groups so as to form 

 small unmixed collections in the main vegetation ; 



copious (with abbreviations cop. 3 , cop. 2 , cop. 1 , to denote decreasing frequency), 

 species represented by individuals scattered in smaller numbers among those 

 previously mentioned ; 



sparse : species having only isolated individuals occurring here and there ; 



solitary : species of which individuals occur in extreme isolation. 



These terms may be used in combination ; for instance, solitary gregarious 

 would mean a single clump composed of one species. The relative shares normally 

 taken by the various plants constituting a community ought to be capable of 

 numerical expression.* 



B. FORMATIONS 



_ It is not sufficient merely to distinguish among the broader, physiogno- 

 mical types, for between these there occur differences that demand 

 the establishment of subdivisions. In attempting to define these many 

 difficulties are encountered. 4 



The term 'formation', or 'vegetative formation', was introduced in 1838 

 by Grisebach in the form ' phytogeographical formation ', which subse- 

 quently gave place to ' vegetative formation '. Grisebach wrote, 5 ' I give 

 the name phytogeographical formation to a group of plants, such as a 

 meadow or a forest, that has a fixed physiognomic character. It is 

 characterized sometimes by a single social species, sometimes by a com- 

 plex of dominant species belonging to one family, or it exhibits an aggre- 

 gation of species which, though diversified in organization, yet have 

 some feature in common, as is the case with alpine meadow-wastes 

 consisting nearly exclusively of perennial herbs.' 



Other writers have, however, attached a narrower signification to the 



1 See p. 93 and Chap. XCVIII. 2 Drude, 1888, 1889, 1895. 



:i Compare Clements, 1905. * See Chap. III. 5 Grisebach, 1838 ; 1880, p. 2. 



