Studies in Prairie and Woodland 19 



ending May 18. The difference between the two curves is slightly 

 in excess of ten degrees, but the daily fluctuations of the two 

 curves are practically of the same amplitude, the two being in 

 general, of course, higher than in April. This interesting corre- 

 lation is probably due to the fact that at that time the trees and 

 other species in the woods were not yet leaved out sufficiently to 

 shade the soil to any great extent. The soil in the woods was 

 exposed to nearly full insolation during mid-day almost as in the 

 prairie, but the period of maximum temperature occurs at a 

 slightly earlier time of day and does not last quite so long as in 

 the prairie. 



The curves for the week ending on June 18 show some very 

 striking differences as compared for the week in May. In the 

 first place both curves show the expected elevation, being on the 

 average about ten degrees higher than the respective curves in 

 the middle of May. But the June curves exhibit a great dif- 

 ference in amplitude or fluctuation. The curve from the woods 

 is almost flat, due to the fact that the soil was completely shaded 

 and although the temperature is considerably higher the fluc- 

 tuations from day to day and from day to night are relatively 

 slight. The curve from the prairie, on the other hand, shows 

 great diurnal and nocturnal variations. These conditions con- 

 tinue into July, but with the temperature in the woodland soil 

 approaching more nearly that of the prairie. During the week 

 ending July 16 the nocturnal minima shown by the two curves 

 very nearly coincided, but the maxima were still far apart due to 

 obvious reasons. The soil temperature at the woodland station 

 remained about the same in August as it was in July varying 

 through only about five degrees about 70° from day to day, but 

 the soil temperature at the prairie station reached a point nearly 

 ten degrees higher than in July with about the same daily fluc- 

 tuation. 



The most significant features exhibited by the march of soil 

 temperature at a depth of 2 in. for the entire season in the two 

 sites are the greater amplitude and the higher extremes of tem- 

 perature in the prairie station. The graphs clearly portray the 

 temperature relations surrounding germination or other growth 



25 



