72 



winds to brings moisture inland,* and the Great Lakes region adds its 

 quota. 



In the storm-track zone, where stagnation of the air is due largely 

 to the balance existing between the continental low and the oceanic 

 high, the aridity of the plains extends the farthest east, and as an arid 

 peninsula it crosses Illinois, giving during August a relative humidity 

 to the prairie area of 60-70 per cent, of saturation (Johnson, '07). 

 The reality of the arid peninsula across Illinois is further shown by 

 the rainfall-evaporation ratios computed and mapped by Transeau 

 ('05). These ratios were determined by dividing the mean annual 

 rainfall at each place by the total mean annual evaporation. These 

 mapf)ed percentages show that the prairie region is closely bounded 

 by the region with an evaporation ratio of between 60 and 70 per 

 cent, of the rainfall received. These conditions furnish a general 

 background or perspective for a profitable consideration of the local 

 and more detailed studies which have been made of the relative evap- 

 orating power of the air in different plant and animal habitats. 



For our purpose it is not necessary to consider the history of meth.- 

 ods of measuring relative evaporation. This measurement may be 

 made by evaporating water in open pans or by the porous porcelain-cup 

 method. Such cups have been devised by several students, but a modi- 

 fied form of the Livingston atmometer has been mainly used by plant 

 ecologists, and this was the kind we used at Charleston. Transeau 

 ('08) was the first to use such an instrument and to show its value in 

 studying the relation of intensity of evaporation to plant societies. 

 His work on Long Island, N. Y., showed very clearly that evaporation 

 in open places was much greater than in dense forests. These obser- 

 vations were enough to show that evaporation is a factor related to the 

 physical conditions of life upon the prairie and in the forest, and there- 

 fore in our cooperative study of the Charleston area in 1910 relative 

 evaporation was made a special feature in the study of representative 

 environments, in order to determine its relation to both the plants and 

 the animals. So far as is known this is the only study yet made in 

 which these determinations have been recorded from the same places 

 where the animals have been studied. Since our dat^. were secured, 

 several papers have been published on relative evaporation in different 

 sorts of habitats in this state and in northern Indiana by plant ecolo- 

 gists Fuller ('II, '12a, '12b), McNuttand Fuller ('12), Fuller, Locke, 



*Zon ('13) has recently asserted that the moisture from the sea does not 

 make a single overland flight inland, but rather is largely precipitated near the 

 sea, is evaporated and carried farther inland,^ is precipitated again, and this 

 process repeated again and again, so that its inland flight is a vertical revolv- 

 ing cycle of precipitation and evaporation. If this contention is valid, evapo- 

 ration from the land is a much more important climatic factor than it is usually 

 thought to be. 



