2 THE PLANT WORLD. 



the subterranean water stands at or near the soil surface, the 

 action of evaporation is directly upon the organism, for this 

 element does not then affect soil moisture. But in most 

 localities a high evaporating power of the air is accompanied 

 by a low moisture content of the soil, unless great rainfall 

 provides new moisture to replace that which escapes. In this 

 connection it must be noted that regions of great precipita- 

 tion are apt to be regions of low evaporation, the low evapo- 

 ration rate being usually accompanied by high humidity and 

 this being, in a sense, both the cause and the effect of great 

 precipitation. 



While the amount of rainfall is a condition effective 

 upon soil moisture, and hence upon vegetation, over large 

 areas, the evaporating power of the air varies greatly within 

 such areas, and contiguous habitats may exhibit marked differ- 

 ences in this power. Two atmometers, one in sunshine and 

 the other in shade, always give very different readings, as 

 would logically be expected from the nature of evaporation. 

 Furthermore, the condition of the soil, nearness to water 

 surfaces, presence or absence of spray in the air and of 

 unusual amounts of saline materials in the soil, exposure to 

 wind, et cetera, all influence the evaporating power of the 

 air to a marked degree. To obtain direct evidence as to the 

 relation of the rate of evaporation to the nature of the vege- 

 tational cover in nearby habitats, and to obtain information 

 on the value of the atmometer in differentiating such habitats, 

 several tests have been carried out during the recent growing 

 season by a number of observers scattered over the United 

 States. This work was done in connection with another line 

 of inquiry, carried out by the Desert Laboratory in co-opera- 

 tion with the observers, and the results of the comparative 

 tests at the majority of the stations where these were carried 

 on will be brought out by the observers themselves. Data 

 from a set of two atmometers at the Missouri Botanical 

 Garden, and from another set of two at Columbia, Missouri, 

 will be here presented. Also, the writer was able to obtain 



