22 THE VEGETATION OF THE UNITED STATES. 



forms are carried on into our classification of vegetation. The fact 

 that the water-relations of plants are more easily known from external 

 criteria, and the fact that they have been more thoroughly investigated, 

 have not only influenced our prevailing system of growth-forms, but 

 have determined the nature of our vegetational units. 



The classification of growth-forms and the classification of vege- 

 tation are like all other scientific efforts to reduce natural phenomena 

 to a logical system, in that the classification possesses its chief value 

 as a concise expression of the results of research. A classification of 

 growth-forms which had been highly perfected by our present methods 

 and knowledge would still be roughly made from the point of view of 

 the ecologist and phj^siologist of tomorrow. It is perhaps idealistic, 

 and is surely premature, to hope that we may one day have an eco- 

 logical classification of the vegetable kingdom on a physiological 

 basis. Such a classification will merely be the perfecting of the begin- 

 ning which has been made by Drude and his predecessors. It will not 

 be possible without a great deal of physiological work that is not yet 

 so much as planned, and it will not be of more than academic interest 

 unless it is constructed from a broad ecological point of view. 



V. PLANT COMMUNITIES. 



The study of vegetation is essentially a study of plants which are 

 growing together in a state of nature, it is an investigation of all the 

 phenomena which these plants exhibit as an aggregation, as dis- 

 tinguished from the behavior of any one of them when considered 

 alone. The natural assemblages of plants which characterize given 

 areas have been assiduously studied by a very large number of workers 

 in all portions of the world. The contrast between the small aggre- 

 gations of local character, the larger ones of more general occurrence, 

 and the still larger ones of very wide distribution has given rise to the 

 recognition of various ranks of aggregation or association and to the 

 study of the relationship existing between aggregations of different 

 rank. By common consent among plant geographers and ecologists, 

 the term "community" has been adopted as a general designation for 

 any assemblage of plants regardless of its rank in the formal schemes 

 of classification. 



In our work with the vegetation of the United States we have had 

 to do with the climatic conditions influencing certain of the plant 

 communities, and we have been under the necessity of deciding upon 

 the criteria to be used in differentiating the communities, as well as 

 under the need of disregarding, for our immediate purpose, certain 

 other communities which stand in definite relation to climatic con- 

 ditions, as well as the communities which are affected by cHmate 

 chiefly through the medium of the soil. We have, perhaps, been some- 



