Nov., 1911.] Climatic Conditions and Plant Growth. 389 



From the tabulation it is seen that the year 1908 was 1.4 

 degrees warmer than 1909; but what is more to the point, the 

 wami gro'^A'ing months from April to September inclusive averaged 

 2.8 degrees wamier in 1908 than in 1909. This higher temper- 

 ature made the drought more disastrous for plant life. 



A porous cup atmometer was operated near Oxford, by 

 Professor S. R. Williams, through the months from June to Sep- 

 tember, 1908, inclusive. The work was done for Messrs. Burton 

 E. Livingston and Forrest Shreve, who have kindly given us the 

 figures for use. Without correcting for depth, the figures are 

 valuable in showing the atmospheric conditions under which 

 vegetation existed here for these months, and in making possible 

 comparisons with those obtaining in other portions of the country. 

 The porous cup atmometer records the evaporating power of the 

 air as this affects the water layer covering the moist clay surface 

 of the cup. This surface is in many respects comparable to that 

 offered by transpiring foliage under the influence of air conditions. 

 The cups were operated during the same months at a large number 

 of selected stations in various portions of North America, and 

 the results obtained near Oxford may be compared readily with 

 those found elsewhere. The average weekly evaporation in the 

 vicinity of Oxford was as follows: June, 1.32 cc. ; July, 182 cc; 

 August, 211 cc; September, 212 cc. The evaporation for June 

 over the region east of the 100th meridian was somewhat more 

 than 100 cc. per week. In July there was a local area covering 

 northeastern Ohio, eastern Michigan and a large part of Penn- 

 sylvania and New York that showed a weekly average of about 

 200 cc. The conditions remained about the same over this area 

 during August. Comparison proves that we were, during all of 

 this time, in a region of very dry atmospheric conditions where 

 the evaporation was high, compared with records for other portions 

 of eastern North America. 



The evaporation from the porous cup atmometer is independent 

 of soil moisture and depends upon the atmospheric conditions. 

 Therefore, conditions of soil moisture can not be deduced from 

 evaporation figures, but must be worked out separately. Our 

 study of soil moisture conditions began early in October, 1908, 

 when the drought was at its height and the cumulative effect on 

 the soil was marked. The study was continued for nine months, 

 extending into July, 1909. The results would have been more 

 valuable, had the study begun five months earlier. Below is 

 given the table of soil moisture: 



