EVAPORATION AND PLANT HABITATS. 3 



some comparative atmometric records from open exposures 

 at Tucson and at three difi'erent altitudes in the Santa Cata- 

 lina mountains, which will likewise be given in this paper. 



The atmometer here used was of the form described in 

 the paper on Evaporation and Development, and the instru- 

 ments were placed as there described, so that they gave read- 

 ings on the evaporating power of the air at a height of about 

 fifteen centimeters above the soil surface. 



The data from the Missouri Botanical Garden were ob- 

 tained through the kind cooperation of the Director of the 

 Garden and of Mr. Henri Hus. This comparison was plan- 

 ned to exhibit the relation between the evaporating power 

 of the air in the open sunshine and that in the shade of a 

 deciduous coppice. The instrument for sunshine was placed 

 in the midst of an herbaceous garden, and was surrounded 

 by a denuded area with a radius of about two meters. It 

 was not in shade at any time of day, and all vegetation was 

 kept from the area throughout the season. The atmometer 

 for shade conditions stood in a coppice of deciduous shrubs 

 and small trees in the arboretum, and was surrounded by the 

 natural carpet vegetation of the region. It received only a 

 small amount of sunshine during the day. 



Two cups were selected with practically the same coef- 

 ficient of correction and these were placed at St. Louis so that 

 the records began on May 20. These continued in operation 

 for four weeks, till June 17, when an accident resulted in the 

 breaking of the shade instrument. A third cup, the original 

 standardization of which gave practically the same coefficient 

 of correction as the other two, was then sent to St. Louis and 

 there installed for two weeks beside the remaining instru- 

 ment of the original pair, after which it was placed under 

 the coppice, and the observations continued. In the second 

 standardization we have a test of the evaporation rates of 

 two cups under identical conditions, the one unused except 

 for the original standardization, and the other used for six 

 weeks (two weeks having elapsed between the breaking of 



