THE WEIGHT OF PHYSICAL FACTORS IN THE STUDY 

 OF PLANT DISTRIBUTION! 



FORREST SHREVE 

 The Desert Laboratory, Tucson, Arizona 



We have in our possession a truly prodigious and rapidly 

 growing body of facts relative to the distribution of terrestrial 

 flowering plants. These facts have been recorded, arranged, and 

 classified by students of plant geography, and many men have 

 yielded to the essential scientific impulse of attempting to explain 

 them. The task of . explaining the distribution of plants is one 

 of such immensity, such complexity, and of such wide depend- 

 ence on related sciences that its progress is a prolonged and slow 

 one, working through a multitudinous array of facts toward the 

 ultimate goal of general principles. 



The search for underlying causes in plant geography may 

 concern itself with the total ranges of plants or with the local 

 or habitual occurrence of these plants within their areas. The 

 student of general distribution has been concerned with the 

 delimitation of floral regions and the evolutionary history of their 

 species, and has invoked his aid chiefly from paleobotany. The 

 student of local distribution has been concerned with physio- 

 logical or ecological aspect of the subject, and has called to his 

 aid all of the physical sciences. 



So sharply drawn has been the line between th^ study of dis- 

 tribution as a floristic subject and its study as an ecological sub- 

 ject that we have been prone to lose sight of the commanding 

 and obvious features which are common to these adjacent lines 

 of work. 



The surface of the earth presents a large number of dissimilar 

 climates, merging into one another, and an enormous number 



^ Paper read in the symposium on Geographical Distribution at the meeting 

 of the Botanical Society of America at the University of California, August 

 5, 1915. 



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THE Pr.A.VT WORLD, VOf.. 19, xo. 3, 1916 



