66 



THE ECOLOGICAL RELATIONS OF ROOTS. 



they are probably not developed so extensively because of the greater 

 available water in the deeper soils. This matter is further discussed 

 on pages 79 and 88. 



A continuous record of the soil temperature was obtained at a depth 

 of 4 inches among these roots from June 5 until August 5. At this 

 time the bulb of the soil thermograph w*as lowered to 8 inches. The 

 most interesting fact disclosed was the extreme variation in tempera- 

 ture during the 24-hour period. This ranged from 60° or 70° to 90° 

 or %° F. or even more. During August, at a depth of 8 inches in the 

 soil, the daily fluctuation was usually about 10° F. within the range of 

 70° to 85°. 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. Table 16 gives the mean monthly temperatures for 

 Colorado Springs. 



Table 16. 



Time. 



Temp. 



Time. 



Temp. 



Jan 



28.0° F. 



Aug 



66.9° F. 



Feb ,,, 



29.8 



Sept 



59.3 



Mar 



37.0 



Oct 



48.4 





44.8 





37.5 



May 



54.1 



Dec 



30.6 



June .... 



63.4 







July.... 



67.9 



Aver . . . 



47.3 



The humidity is relatively low. During clear days, excepting early 

 in the morning, it is usually less than 50 per cent and it not infre- 

 quently drops to 10 or 15 per cent and sometimes lower. Even at 

 night, when the air is coldest, it seldom exceeds 80 or 90 per cent, and 

 dew rarely forms. There is usually steady wind movement on the 

 plains. Records from a standard anemometer placed just above the 

 plants at a height of 0.5 meter show an average daily wind velocity of 

 120 miles during the growing season of 1918. Likewise daily water- 

 losses from non-absorbing atmometers placed at a height of 10 cm. 

 above the soil surface are relatively high. From the data given in 

 table 17, which shows the average daily evaporation on the plains, 

 it may be seen that the plains habitat is xerophytic as regards both 

 air and soil environment. 



In order to visualize clearly the actual root position and the con- 

 ditions of competition, the accompanying bisect (plate A) was made. 

 Similar bisects are also given for other plant communities. The 

 method used is the same in all cases. A representative meter-quadrat 

 was selected which showed both dominant and subdominant species. 

 After charting and photographing, a trench was dug in front of the 

 quadrat, care being taken to have the wall corresponding with a side 



