ROOT SYSTEMS OF TRUE-PRAIRIE SPECIES. 



33 



growth both by night and by day. The wide daily fluctuations in 

 temperature at the mixed-prairie station, especially the relatively low 

 temperatures at night, are certainly less favorable to plant growth 

 than are the conditions prevailing in the true prairie. At the Colorado 

 Springs station the daily fluctuation of air temperature among the 

 plants was usually about 35° to 40° F., the air reaching a maximum of 

 90° or 95° F. in the shade in the afternoon and falling to 50° or 60° F. 

 in the morning. 



•F 



95 



90 



es 



80 

 75 

 70 

 65 

 60 

 ■55 

 50 



AS 



Fig. 9. — Graphs showing the average weekly day and night tem- 

 peratures in the mixed prairie (broken lines) and true 

 prairie (solid lines) during 1919. 



According to Weather Bureau records, the season without killing 

 frost usually extends from April 19 to October 10 at Lincoln, and 

 from May 3 to October 3 at Colorado Springs. The Lincoln sta- 

 tion is at an elevation of only 1,200 feet, that at Colorado Springs 

 at 6,000. However, it seems quite certain that water-content of 

 soil and air, and not temperature, are by far the principal factors in 

 plant development. Further study will probably show that growth 

 is often temporarily suspended for some mixed-prairie species, and 

 perhaps permanently for others, by more or less extended periods of 

 drought. 



While soil temperatures were not obtained synchronously at the two 

 stations, a comparison of the records taken at a depth of 4 and 8 inches 

 at Colorado Springs during 1918 with those at Lincoln during 1916, 

 1917, and 1919 shows some marked differences. Those in the mixed 

 prairie show an extreme variation in temperature during the 24-hour 

 period. This ranged from 60° or 70° F. to 90° or 95° F. or even more 

 at the shallower depth. During the latter part of the summer, even 



