32 



ROOT DEVELOPMENT IN THE GRASSLAND FORMATION. 



figure 8, where the data from the true-prairie stations at Lincoln, 

 Nebraska for 1916 and 1917 are plotted with those obtained from the 

 mixed prairie at Colorado Springs, Colorado, during 1918. At both 

 stations the day losses are by far the greater. A consideration of the 

 lower temperature of the mixed prairie at night in contrast with the 

 conditions in true prairie, emphasizes the probability that the evapora- 

 ting power of the air during the day in mixed prairie may exceed that 

 of the true-prairie community even to a greater degree than the graphs 

 indicate. The effect of excessively dry air, coupled with low water- 



Fig. 8. — Graphs showing the average daily evaporation rates in the 

 mixed prairie (1918) and true prairie (1916 and 1917). 



content, accounts for the absence here of many true-prairie meso- 

 xerophytes and the relative dwarfness of many true-prairie species 

 when growing in the mixed-prairie habitat. 



The average weekly day and night temperatures at the two stations 

 during 1919 have also been determined. These data were obtained 

 from the weekly record-sheets of thermographs placed in appropriate 

 shelters among the vegetation at a height of about 4 or 5 inches. The 

 averages for day temperatures were found by adding the temperatures 

 beginning at 8 a. m. and every 2 hours thereafter until 6 p. m. for each 

 day and dividing the sum by the total number of 2-hour intervals. 

 Those for night temperature were calculated in a similar manner, 

 beginning at 8 p. m. and including the readings until 6 a. m. An ex- 

 amination of the temperature graphs in figure 9 shows that, with few 

 exceptions, the true prairies have a more favorable temperature for 



