INTRODUCTION. 23 



what informal in our handling of this eminently formal subject, upon 

 which so much has been written and so much has been enacted by 

 botanical congresses. 



As already intimated, the study of vegetation has resulted in the 

 recognition of different degrees of communal existence among plants. 

 These degrees have been designated by names, among which may be 

 found the words formation, region, zone, society, association, district, 

 consocies, group, belt, strip, and a score of others. Scarcely any two 

 workers have used the same term in precisely the same sense, and few 

 of the workers have defined their terms in such a manner as to enable 

 a botanist to recognize one of the communities in case he should find 

 himself in its midst. There has been an organized effort in recent 

 years to secure a general and international agreement regarding the 

 classification and nomenclature of plant communities. The extensive 

 areas such as the sagebrush plains of the Great Basin, the grasslands 

 of Nebraska and Kansas, or the pine forests of the Atlantic Coastal 

 Plain are designated as formations. The smaller and less markedly 

 differentiated areas within a formation are designated as associations, 

 as, for example, the forests of shortleaf pine in New Jersey, those of 

 loblolly pine in Maryland and Virginia, and those of longleaf pine in 

 the Gulf States, all lying within the Coastal Plain formation. The 

 smallest units of vegetation are termed societies, and these are of small 

 area and represent portions of the association in which a definite 

 aggregation of species is to be found. 



This classification of communities is simple and natural and has 

 much to commend it for general use in describing vegetation. It is to 

 be noted that, just as the formation is defined in terms of growth- 

 forms which are found to be most common and characteristic in that 

 community, so the association is defined partly in terms of the growth- 

 forms present and partly in terms of species, while the society is 

 defined chiefly by the species which it contains. 



The most important criterion to be employed in the distinguishing 

 of communities is always the kind or kinds of growth-forms which are 

 present, and this is a criterion which can be used for societies as well 

 as for formations. A community may present a single growth-form, 

 represented by a single species or by a group of species, as is true of 

 many sahne marshes and of very many forest areas. It may present 

 an intermingling of two or three growth-forms, as is true of those saline 

 marshes that contain grasses, the succulent Salicornia and the large- 

 leaved Statice. In certain localities there occur communities which 

 are made up of a very large number of growth-forms, as, for example, 

 in the Karroo Desert of South Africa and in the deserts of Tehuacan, 

 Mexico. 



Communities may be of such a character as to contain only plants 

 of a single order of size, as the short-grass prairies of Nebraska, or they 



