INTRODUCTION. 7 



dissimilar trees often grow side by side. We know that arborescent 

 cacti and microphyllous trees differ markedly in the character and 

 activity of their root-systems, and that the annual course of absorption 

 and transpiration in the two types is very unlike, in spite of the fact 

 that the two are constant associates. In view of the incompleteness of 

 the knowledge which might afford a basis for the classification of life 

 forms, it behooves us to recognize a limited number of such forms and 

 to extend the list only on securing evidence of the divergent behavior 

 of groups of plants. 



The difficulties inherent in the classification of vegetation have led 

 some of the German and British botanists to use the physical charac- 

 teristics of the habitat either as the sole criterion for classification or 

 else as a secondary criterion, employed together with the charac- 

 teristics of the plants themselves. Such a procedure may be looked 

 upon as an indirect method of securing evidence of the physiological 

 cliaracter of the plants involved and is logically allowable only on such 

 ground. In any case in which plants of identical growth-form occur 

 in two situations of very unlike physical conditions, an excellent 

 opportunity is afforded to investigate the comparative physiology of 

 the plants. If, as is very unlikely, the two groups should be found to 

 be physiologically alike, there would then be no ground whatever for 

 the separation of the two vegetations in classification. If physiological 

 distinctions should be discovered, these, rather than the unlikeness of 

 the habitats, should then form the basis for separating the two vege- 

 tations. 



Our deepest concern for the development of plant geography is 

 that its activities may be diverted from the description and classifica- 

 tion of vegetation on subjective grounds and that they may be directed 

 toward experimental work so planned as to yield an actual physio- 

 logical basis for the classification of vegetation. Starting with the 

 small body of experimental data which now makes it possible to recog- 

 nize certain groups of plants with a general coherence of physiological 

 behavior, it will become possible in the course of time, and of much 

 hard work, to extend our knowledge far enough to actually under- 

 stand the different types of vegetation which we now photograph, 

 map, fist, and name. 



II. STUDY OF THE DISTRIBUTION OF INDIVIDUAL SPECIES. 



The investigation of correlations between climatic conditions and 

 vegetation has been extended, in our work, to the relations between 

 the climate and the distribution of certain individual species of plants. 

 We are here treading upon fresher ground, on which extremely Httle 

 work has been done. To attempt to define the factors which are 

 responsible for the geographical distribution of any one plant species 



