66 



Development and Activities of Roots of Crop Plants. 



ditions are very favorable for the conservation of water by plants. During 

 these periods the wind, an important ecological factor, was still or of low 

 velocity. Normally it is quite pronounced, averaging 130 to 160 miles per 

 day and two to four or more times this amount during very windy weather. 

 It plays an important part in desiccating both plants and soil. Throughout 

 the period the evaporating power of the air ranged from 23 to 39 c. c. daily 

 (table 18). Owing to the cool nights, which cause a rise in humidity, coupled 



Fig. 29. — Average day humidity (lower lines) and average night humidity 

 (upper lines) at Lincoln (solid lines) and Burlington 

 (broken lines), 1920. 



with the lesser wind movement, the evaporating power of the air is low. For 

 example, from June 28 to 30, 1921, with typical clear, hot days and cool nights, 

 the average day evaporation (6 a. m. to 6 p. m.) was 48 c. c. with 104 miles of 

 wind, while the average night evaporation was only 10 c. c. with 51 miles of 

 wind. Thus 83 per cent of the evaporation occurred during the day period. 

 The conditions of low available water, high soil and air temperatures, es- 



Table 18. — Average daily evaporation at Burlington, 1920. 



June 3 to 11 46.9 



June 11 to 18 34.8 



June 18 to 25 22.4 



June 25 to July 2 38.4 



July 2 to 8 38.6 



Julys to 15 42.1 



July 15 to 20 31.8 



July 20 to 27 44.2 



July 27 to Aug. 5 26.0 



Aug. 5 to 15 24.2 



Aug. 15 to 18 69.8 



Aug. 18 to 25 22.6 



pecially by day, combined with great wind movements and low humidity, 

 caused not only a dwarfing of the above-ground plant parts, but resulted 

 in an extensive development of the root systems. 



The wheat roots were very abundant to the depth of the hardpan, 2.7 

 feet. This dry layer determined the maximum penetration, which had 

 not increased since the last examination, although the working depth was 

 now only a little less than the maximum. Likewise, the lateral spread had 



