A Study of Hvaporation and Plant Distribution. 221 



region at about 350 cc. per week. The reciprocals of these rates 

 are 1.00, 0.57, 0.29, to compare with 1.00, 0.84, 0.16, from our 

 summer approximations of 1907, and with our approximations 

 from Transeau's annual ratios, 1.00, 0.86, 0.35. It is ai)parent 

 that the agreement between the annual index (from Transean's 

 ratios) for the deciduous forest center (the index for the north- 

 eastern conifer area being assumed as unity) is in much closer 

 agreement with that for oar first summer approximation than 

 with that for 1908, while the annual index for the desert region 

 agrees more satisfactorily with our summer index for 1908 than 

 with that for the previous summer. Such a condition seems to 

 exhibit the low degree of detailed accuracy which may be 

 possessed by any method which depends upon climatic data for 

 a single year; it is practically certain that such indices as we have 

 used must fluctuate between wide limits in difl'erent years. 

 Transeau's ratios, while calculated form the data of mean 

 annual precipitation based upon observations for many years, 

 also involve the evaporation rates for a single year, as obtained 

 by Russell in 1887 and 1888, The time for nice quantitative 

 distinctions and critical studv in these matters has obviously 

 not arrived. 



From the above general deductions it is apparent that the 

 summer evaporation intensity alone furnishes a climatic criterion 

 for studying the different vegetation centers with which we have 

 to deal at least as promising as the ciiterion furnished by any 

 other meteorological element. Summer evapoiating intensity 

 while exhibiting relations not in detailed agi cement \\ith those 

 shown by the annual rain-evaporation ratios of Transeau, may 

 be considered of as great promise in this general problem as are 

 the latter; the fact that the annual ratios and the two series of 

 summer data are all for different years might give as discordant 

 results as we have obtained, even though the mean intensities 

 for an adequate period of years might still show agreement. 

 The summer data, especially if obtained by some such means as 

 ours, offer much less difliculty in their derivation than do those 

 used for the annual ratios, so that theie may be more hope of 

 getting an adequate series of them to extend over a number of 

 years. The importance of the relative intensities of evaporation, 

 to ecology and agriculture, are only surpassed by the present 



