ipu] OF THE UNITED STATES. 271 



Thus, heavy clays occur comiiionl\- throughout the United States 

 and the moisture relation of plants growing thereon may be aijproxi- 

 mately proportional to the relation of precipitation to evaporation. 

 A similar proposition may hold for sandy soils. It is, however, to 

 be noted that a sandy soil and a clay soil under the same climatic 

 conditions ought not to be expected to possess the same power of 

 supplying moisture to plants. 



The relation of precipitation to evaporation was first emphasized 

 as a climatic factor influencing vegetational distribution in the United 

 States by Transeau,^- who constructed a very interesting and valuable 

 chart of the eastern portion of our country on the basis of the ratio 

 of mean annual precipitation to the annual evaporation obtained by 

 Russell for a single year. Another, and in some ways more satis- 

 factory relation between rainfall and evaporation is the difference 

 between these factors, precipitation mi mis evaporation. I have 

 tested this as extensively as our extremely meager data on evapora- 

 tion will allow. In the present paper attention will be confined to 

 this index of difference for the frostless season. 



We now turn our attention to three examples of the quantitative 

 study of the moisture relations of the United States, resulting in the 

 means of precipitation, of evaporation and of the difference between 

 these two for the frostless season. 



2. Amount of precipitation during the frostless season. — Bige- 

 low has given us, by means of very ingenious and elaborate methods. 

 a table showing the daily normal precipitation for each of 177 sta- 

 tions in the United States, and it is upon this valuable work that I 

 have based all of my quantitative studies of rainfall. In the present 

 instance, wherein the normal distribution of precipitation during the 

 year will receive no attention, I have merely determined the average 

 normal daily precipitation at each station throughout the frostless 

 season. This gives a precipitation index which is at once seen to be 

 .definitely related to plant activities. Stations with high precipita- 

 tion indices are situated in the humid regions, those with low indices 

 are in the arid regions. 



"Transeau. E. N., "Forests of Eastern America." Amer. Nat.. 39: 

 875-98, 1905; "Climatic Centers and Centers of Plant Distribution," Mich. 

 Acad. Sci. 7th Ann. Rept., 1905. 



