INTRODUCTION. 21 



forms. If we wish to understand vegetation we must understand the 

 individual species of whicji it is composed. If we wish to understand 

 the relation of each plant species to its environment we must under- 

 stand the nature of its functions and the character and role of each of 

 the organs through which they are carried on. Whatever features of 

 the gross anatomy of plants may be discovered to have no apparent 

 importance in any aspect of their adjustment to environment will have 

 no place in shaping our ultimate system of growth-forms. Progress 

 toward such an ultimate system is beset by two dangers: that which 

 would lead us to be satisfied with a system which is too simple, and 

 that which would lead us to adopt a system in which anatomical 

 features of questionable importance would be recognized along with 

 those of demonstrated importance. 



Vegetational units have been grouped or classified by various 

 workers according to the nature of the habitats in which they are found, 

 according to their floristic make-up, and according to their successional 

 relation to one another. A large body of work has been done by these 

 methods, giving us a substantial part of our knowledge of the vegeta- 

 tion of the globe. The only one of these methods which is purely 

 vegetational is the last. If it were possible to demonstrate changes 

 of vegetation in a state of nature which were not accompanied by 

 changes of environmental conditions, it would indeed be necessary to 

 give strict attention to the stages of succession in making any attempt 

 to correlate vegetation and conditions. If it were true that identical 

 conditions might sometimes present different vegetations, our problem 

 of correlation would be made still more complicated than it already is. 

 It has been amply shown, however, that successional changes of vege- 

 tation are both preceded by and accompanied by changes of environ- 

 ment. The well-known work of Cowles has shown the importance of 

 the changes of soil-moisture which accompany physiographic develop- 

 ment, and the work of Fuller, of Gates, of Weaver, and of Cooper 

 has shown the importance of other conditions of both soil and atmos- 

 phere. In addition to the physiographically initiated changes in the 

 environment are those initiated by the vegetation itself, supplying 

 conditions favorable for invasion by a new group of species. Although 

 a large amount of work has been done in describing successions and in 

 relating successional stages to each other, it is only recently that the 

 workers just cited have made a beginning in the investigation of the 

 physical conditions which underlie the separate stages. As soon as we 

 begin to study the relation of physical conditions to successional 

 stages, the relation of these stages to each other sinks to a position of 

 minor importance, and our work emerges upon the broad field of 

 causational plant geography. 



The imperfections of our present knowledge of the physiology of 

 plants and the consequent imperfections of our system of growth- 



