112 THE PLANT WORLD 



If we assume, as unity, the evaporating power of the 

 air, in summer, over the northeastern center, then the index 

 of evaporation for the deciduous forest becomes 1.15, and 

 that for the desert becomes 2.86. On Transeau's map the 

 northeastern center appears to be represented by a precipita- 

 tion-evaporation ratio of about 1.25, while the deciduous 

 forest is represented by a ratio of about 1.05. From the 

 western portion of the map it is apparent that the desert 

 ratio of the Southwest, must approximate 0.30. For the 

 three regions our indices are: 93, 107, and 266, while 

 Transeau's ratios are: 1.25, 1.05, and 0.30. An inspection 

 of these numbers brings out the fact that they represent 

 two very similar series, and that the indices are approximately 

 inversely proportional to the ratios. That is, we have : 

 1.25 : 1.05=1.15 : 1. 00, or 1,25^.97 

 1.25 : 0.30=2.86 : 1. 00, or i.25=.68 



When it is remembered that Transeau dealt with the 

 evaporation rate for the entire year and that we have con- 

 sidered it only for the summer, the condition of agreement 

 between the two methods must be regarded as very satis- 

 factory. It would thus appear that, as far as the present 

 data are to be relied upon, the summer evaporating power 

 of the atmosphere seems to offer as satisfactory a criterion 

 for relating these three vegetational centers to climatic fac- 

 tors as do the precipitation ratios of Transeau. In this 

 connection it is to be remembered that, generally, evaporation 

 is a complex factor, including effects of temperature, 

 humidity, wind v^elocity, rainfall, barometric pressure, and 

 even light intensitv. It thus furnishes a rather satisfactory 

 automatic summation of all the climatic factors which affect 

 the activities of plants. For details of distribution, temper- 

 ature, precipitation, light intensity, etc., will undoubtedly 

 have so kindly attended to the records of the atmometers at 

 the region and of the plant forms involved. 



I wish to express here my thanks to the observers who 

 have so kindly attended to the records of the atometers at 

 the several stations. Without their interest and aid this 

 study could not have been undertaken. 



Munich, March 26, 190S. 



