EVAPORATION AS A CLIMATIC FACTOR. 53 



evaporation conditions in sunny and shady situations have been 

 made during the growing season just past. These tests uni- 

 formly show a much higher evaporation rate in the sun than in 

 the shade. An instance of this difference may be taken from 

 the readings of two instruments installed at the Missouri Botan- 

 ical Garden, at St. Louis. For these data I am indebted to the 

 Director of that Garden, for an active and appreciative interest, 

 and to Mr. Henri Hus, for the care of the instruments and the 

 preparation of the data. In this test one of the evaporimeters was 

 exposed 15 cm. above the soil surface in a denuded area about 

 three yards square lying within the Experiment Garden. From 

 this area vegetation was excluded during the season. The sec- 

 ond instrument stood at the same height above the ground in 

 a coppice in the Arboretum, where the shade was rather dense. 

 For the period from May 19th to June 17th, the average weekly 

 rate of evaporation was 142 cc. in the sun, and 58 cc. in the 

 shade. From July 22d to August 26th, the average rates were 

 187 cc. and 61 cc. Records for the period intervening between 

 these two were lost on account of an injury to one of the instru- 

 ments. 



Attention was called to the importance of evaporation in 

 the general distribution of forest centers by Transeau, in a 

 paper in The American Naturalist, Vol. 39, pp. 875-889, 1905, 

 The same material, in a condensed form, appeared in the Seventh 

 Annual Report of the Michigan Academy of Science, pp. 73-75. 

 These papers call attention to the fact that the distribution of 

 forest centers in eastern America cannot be accounted for on 

 the ground of heat and precipitation. The author presents a 

 map showing the ratio of rainfall to evaporation, expressed in 

 percentages, deriving his evaporation data from the paper of T. 

 Russell in the Monthly Weather Review for September, 1888, 

 and shows conclusively that this map exhibits "climatic centers 

 which correspond in general with the centers of plant distribu- 

 tion. Further, the distribution of grassland, prairie, open forest, 

 and dense forest regions is clearly indicated. . . . This is 

 explained by the fact that such ratios [of rainfall to evaporation] 

 involve four climatic factors which are of the greatest import- 



