14 THE VEGETATION OF THE UNITED STATES. 



species or to a given type of vegetation, and idealistic only in that it 

 would require extension or modification to fit the necessities of any 

 given case. Such a program, briefly outlined, is as follows : 



A. The securing of the distributional facts. 



1. Securing an exact knowledge of the geographical range of the given plant. 



2. Determining the ecological distribution of the plant. 



a. Its region of greatest abundance. 



b. Its region of greatest size. 



c. Its region of most rapid growth. 



d. Its region of greatest productivity. 



e. Its region of greatest cathoHcity of habitat. 



3. Determining the behavior of the plant at the limits of its range. 



a. The character of its limital habitats. 



b. The evidences for its limitation. 



B. Ascertaining the apparent climatic controls on a correlational basis. 



1. Determining the isoclimatic Unes which follow nearest to the geographical Umits 



of the plant form considered. 



2. Determining its habitat behavior with respect to climatic elements discovered in 1. 



3. Determining its comparative behavior at different portions of the periphery of 



its geographical range. 



C. Ascertaining the actual climatic controls by experimentation. 



IV. GROWTH-FORMS OF PLANTS. 



When we undertake to regard the vegetable kingdom from the 

 ecological standpoint, and to investigate the importance of the physio- 

 logical characteristics of plants as related to their distributional fea- 

 tures, it is clear that considerations of phylogenetic relationship 

 become of little importance. If we attempt to arrange the multifarious 

 plant-forms of the earth in a series of groups according to their physio- 

 logical affinities, so as to bring together the plants which have solved 

 the same problems of environmental adjustment in the same manner, 

 we shall have to depart very far from the families and genera of the 

 natural system of classification. 



This is very obvious from a consideration of the diversity that 

 exists in some of the large plant families. In the Compositse, for 

 example, we have trees, shrubs, and herbs, terrestrial plants and 

 aquatics, large-leaved, small-leaved, and leafless perennials, mat- 

 forming or cushion plants, slender climbers, etc. In the southwestern 

 United States we have, conversely, a group of small-leaved or leafless 

 woody perennials which are green-stemmed and richly branched and 

 bear a close resemblance to each other, in so far as vegetative structures 

 are concerned, in spite of the fact that they may belong to entirely 

 different families. Among these plants are Thamnosma montanum 

 (Rutacese), Koeherlinia spinosa (Koeberliniacese), Holacantha emoryi 

 (Simarubaceae) , Canotia holacantha (Celastraceae), and Parkinsonia 

 microphylla (Leguminosse). 



Among the criteria used in the phylogenetic classification of plants 

 are : the structure of the flower, the developmental history of the floral 



