38 Development and Activities of Roots of Crop Plants. 



weekly record-sheets of the thermographs by adding the temperatures begin- 

 ning 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 the 

 night intervals were calculated in a similar manner, beginning at 8 p. m. and 

 including the readings until 6 a. m. The weekly maximum and minimum 

 readings were determined directly by averaging the 7 highest and 7 lowest 

 points respectively, on the weekly record-sheet. This method of evaluating 

 temperatures takes into account both temperature extremes and the element 

 of time. A gradual increase in temperature with the progress of the season 

 is evident. The high temperatures during July and August were very 

 favorable for the rapid development of corn. 



The factors of high temperature, low humidity, and wind movement com- 

 bine to increase the evaporating power of the air and transpiration of crops. 

 In a study of root development it is important to consider the above-ground 

 environment. For just as the possible growth of the aerial parts of plants 

 is affected by the extent of the development of the root system, conversely 

 the environmental conditions to which the aerial parts are subjected, espe- 

 cially as concerns their water relations, must reflect themselves in root devel- 

 opment. Table 7 shows the average daily evaporation losses during the 



Table 7. — Average daily evaporation at Peru, 1919. 



Apr. 21 to 28 5.8 



Apr. 28 to May 7 11.8 



May 7 to 12 20.0 



May 12 to 20 24.0 



May 20 to 28 28.3 



May 28 to June 9 19.0 



June 9 to 16 21.4 



June 16 to 23 27.3 



June 23 to 30 26.6 



June 30 to July 8 20.9 



July 8 to 15 27.0 



July 15 to 22 22.7 



July 22 to 29 44 . 1 



July 29 to Aug. 5 38.4 



several periods from April 21 to August 5. An examination of these data 

 reveals no unusually high evaporation rates when compared with other 

 seasons' (Weaver 1919, 1920), except during the latter part of July and early 

 in August. At this time all of the crops except corn were ripe. During the 

 remainder of August only one or two light showers occurred and the tempera- 

 ture most of the time was quite high (fig. 16). These conditions doubtless 

 reflected themselves in the extensive root development of the corn. 



Summary of Root Development. 



All the cereals, including corn, possessed a root system in which there was 

 a definite group of more or less horizontal, spreading roots lying within the 

 first 1 to 1.3 feet of soil, and a second group of deeply penetrating roots ex- 

 tending into the subsoil to depths of 6 or 7 feet. 



The Early Ohio potato differed from the other plants in that the same group 

 of roots which at the outset formed the shallow portion of the system subse- 

 quently became the deeper portion by turning more or less abruptly to the 

 vertical position and growing downward. 



In every instance the root systems were very extensive in relation to the 

 size of the top. All the main roots were abundantly supplied with branches 



1 Since these atmometers were not fitted with non-absorbing apparatus, direct comparisons 

 of actual losses can not be made but it is certain losses were higher than the figures indicate. 



