206 The Plant World. 



ising ^A•hen considered with the fact that the same general 

 meteorological conditions which promote evaporation tend to 

 render the precipitation low The publication of Transeau's 

 pioneer study * of the relation between vegetation and the 

 precipitation-evaporation ratios for the eastern part of Xorth 

 America gave added interest to this problem, for the charts 

 of this author seem clearly to indicate a very definate corres- 

 pondence between the nature of any vegetation type and the 

 ratio of annual rainfall to annual e\aporation for the region 

 occupied by that type. 



In so far as climatological features control the present dis- 

 tribution of the vegetation of the world, it is to be expected that 

 the quantitatixe relations existing between these features and 

 the vegetaticnal characters will prove to be rather a complex 

 one, and it will no doubt require long continued study to enable 

 us definitely to express these relations. The integrations of 

 many meteorological elements, for various portions of the 

 year, will need to be considered, in the synthesis of the much- 

 desired climatological formula for plant distribution. Since 

 Transeau's (1905) and the writer's (1906) studies on the relation 

 of plant occurrence and behaviour to evaporation intensity seemed 

 to attribute an importance to this particular factor which had 

 never before been emphasized, it seemed desirable that the gen- 

 eral relations between the evaporating power of the air and the 

 nature of the vegetational cover be explored from several 

 different standpoints. It must thus he understood that the 

 present study aims, not at all to displace the method of annual 

 ratios, which was the first recorded attempt to inter] tret plant 

 distribution quantitativeh' in terms of a climatic function in- 

 volving evaporation, but rather to test cne other method for 

 studying the relationship concerned. It was of course to be ex- 

 pected that the two methods, of annual ratios and of summer 

 evaporation intensity, would not exhibit a detailed agreement; 

 the very nature of the problem and of the data involved pre- 

 cludes such agreement, and it is only along broad, general lines 

 that the results of such studies may at the present time be 

 scrutinized. 



*Transeau, E. N.. Forests of Easteu America. Amer. Xat. 39: 875 98. 1905 

 Idem, Climatic centers and centers of plant distribution. Micb. Acad, of Sci., 7th Ann. 

 Report, 1905. 



