22 Development and Activities of Roots of Crop Plants. 



12 per cent. The soil was in fine tilth and warmed up rapidly. The average 

 soil temperature to a depth of 1 foot was 63° F. The above-ground condi- 

 tions were also favorable. The mean daily temperature was about 64° F. 

 The last half of May was clear and dry and the first half of June was cloudy, 

 but warm. The daily evaporation for the period in May was 24 c. c, but 

 for June, due mostly to cloudiness and an increased humidity, it dropped to 

 20 c. c. Thus, the environmental factors, both aerial and within that part 

 of the soil occupied by the roots, were conducive to rapid growth. The corn 

 plants responded favorably to these conditions, as shown by the fact that 

 within a period of 36 days from the date of planting they had reached an 

 average height of 12 inches. 



The second examination of the corn roots was made July 5, 57 days after 

 planting. The stalks were about 4 feet tall and 4 or 5 nodes were visible 

 (plate 1 a). During the period intervening between the first and second 

 excavations, a remarkable extension of the root system had taken place. 

 It now consisted of two rather distinct portions. One of these comprised the 

 original roots, together with those of later development scattered among 

 them and which arose from nodes at very short intervals just above the old 

 kernel. These roots ran out either horizontally or descended very gradually, 

 extending laterally to a distance of 2 to 4 feet from the base of the stalk, where 

 they usually turned downward rather abruptly (fig. 8). They penetrated 

 to a depth of 1.5 to 4.6 feet. The other portion of the root system consisted 

 of a large group of roots of later development that ran almost vertically 

 downward or spread out only a short distance from the base of the plant and 

 then took a downward course. The spread of these roots was seldom more 

 than 10 or 12 inches from the vertical. They were the youngest roots, 

 growing vigorously, and were found in all stages of development. Some 

 were just starting from the base of the plant, while others had already pene- 

 trated to a depth of 4.7 feet. They were succulent and turgid and had a 

 diameter of 3 or 4 mm. or more. Both groups of roots were well supplied 

 with branches, those lying near the surface were most profoundly branched. 

 These branches were in turn often branched and rebranched to the third 

 and fourth orders. The longer branches were confined to the first foot of 

 soil and, with their many ramifications, formed a wonderfully dense and 

 efficient surface absorbing portion of the system. Below the first foot the 

 branches from the main roots were shorter, usually not exceeding 4 or 5 

 inches. In color the roots were either brown or white, depending upon their 

 age. The last 6 to 12 inches of the growing tips were pearly white and devoid 

 of branches. 



This period was more favorable for growth than the preceding. Both 

 air and soil were constantly growing warmer. The mean daily air-tempera- 

 ture was 75° F. The soil-temperature, taken for the first 8 days of the period 

 only, reached 76.3° F. as an average for the first 3 feet by June 23. This was 

 11° F. higher than the reading of June 9. The precipitation was about nor- 

 mal, and the first 3 feet of soil had an average available water-content of 



13 per cent. 60 per cent of the days were clear. The mean daily evapora- 

 tion was 25 c. c. The crop responded to these highly favorable conditions 

 for growth in a remarkable manner. Although the period was but 21 days 

 in length, the corn increased its height approximately 3 feet, an average 



