CLIMATIC AREAS OF THE UNITED STATES AS 

 RELATED TO PLANT GROWTH.^ 



(Plates IX, X, and XI.) 



By burton EDWARD LIVINGSTON. 

 (Read April iS. 1913.) 



Introduction. 



The climatic factors whicli generally determine whether a given 

 kind of plant may or may not live in a certain locality are to be 

 divided into two groups. The first group comprises those factors 

 which tend to increase or to decrease the moisture content of the 

 plant body. They may be termed the moisture conditions of the 

 environment. The second group includes the climatic factors which 

 tend to raise or to lower the temperature of the plant. These are 

 the temperature conditions. A third group of climatic factors in- 

 cludes those tending to increase or decrease the insolation of the 

 plant and hence to promote or retard photosynthesis in green tissues, 

 by which carbon dioxid and water are decomposed with the formation 

 of molecular oxygen and carbohydrate. With these light conditions, 

 however, climatic plant geography has as yet but little to do and 

 this group will not receive attention in the discussions which follow. 



Before plant geographv can pass beyond its qualitatively descrip- 

 tive phase, the moisture and temperature relations that obtain be- 

 tween plants and their surroundings must be subjected to examina- 

 tion much more quantitative than has heretofore been attempted. 

 As in other similar instances, definite knowledge of this complex 

 set of relations can be reached only through measurements of the 

 things that are to be related. It thus appears that, for those chap- 

 ters of plant geography and of scientific agriculture which have to 

 do with climatic conditions, it will presently be found requisite to 



' Botanical contribution from the Johns Hopkins University, No. 32. 



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