1 8 The Botanical Survey of Nebraska 



and woodland stations throughout the vegetative season of 1917, 

 beginning early in April and closing in September. Figure 9 

 shows the weekly march of soil temperature at a depth of 2 

 inches in the prairie and woodland for selected weeks during the 

 months of April, May, June, July and August. The depth of 

 2 inches was selected because it was thought that temperature 

 conditions within that portion of the soil are particularly impor- 

 tant and significant as related to germination and ecesis and con- 

 sequently to the whole problem of succession. Sufficient equip- 

 ment was not at hand to enable us to secure temperature data for 

 other depth classes simultaneously with these secured at a depth 

 of 2 inches. 



The curves in figure 9 show a number of interesting facts about 

 the soil temperature in our two extreme stations. The curves are 

 direct copies of the graphs traced by the thermograph pens upon 

 the record sheets for the weeks ending on the dates indicated. 

 The only change that was allowed in making such transcripts was 

 a slight rounding-ofif of the original graphs to make somewhat 

 smoother curves. This method does not affect the value of the 

 transposed graphs in any significant manner because those ir- 

 regularities were merely sharp angular fluctuations with only 

 slight amplitude, such as may be caused by cloudiness. 



The general and gradual rise of temperature in the two stations 

 is nicely demonstrated by the graphs. The lower graph in all 

 cases is that from the woodland station, the upper one is from the 

 prairie. A striking difference in the amplitude of the two curves 

 is noted after the month of May, the daily extremes being much 

 farther apart at the prairie station, as might be expected. Dur- 

 ing the month of April the amplitude of the two curves is prac- 

 tically the same, even during the last three days of the month 

 when the sky was generally clouded, but all through the month 

 the temperature in the woods was approximately ten degrees 

 lower than that in the prairie. The soil of the prairie warms up 

 much sooner than that in the woods, but the rate of change is 

 about the same during this vernal period. 



The most nearly coincident range of temperature in the two 

 habitats is seen in May as illustrated by the curves for the week 



24 



