24 Development and Activities of Roots of Crop Plants. 



daily growth of 1.7 inches. The plants were sturdy and bore an abundance 

 of dark-green leaves. 



The root system of the maturing corn plants was examined September 2, 

 116 days after planting. The stalks were 8 to 9 feet high and, while a few 

 of the leaves had died, most of them were still green. The husks on the 

 ear were just beginning to dry and the kernels were dented (plate 1 c). The 

 shallow portion of the root system had scarcely increased over that found 

 at the July examination, when the plants were 57 days old (cf. figs. 8 and 9). 

 Most of the roots of this type were found within the first foot of soil, running 

 in all directions from the base of the plant. Some of them maintained a 

 horizontal position throughout their entire course, which sometimes reached 

 a length of 4 feet. Others ran off at various angles to distances varying from 

 a few inches to as much as 3.7 feet and then turned downward either abruptly 

 or with a gentle curve. They then pursued their irregular course downward 

 to a depth of 2.5 to 3 feet. This surface portion of the root system was 

 profusely branched to the second and third order with branches ranging 

 from less than an inch to 2 feet in length. The surface soil to a depth of 8 

 to 12 inches was literally filled with these fine rootlets. 



Unlike the shallower portion of the root system, the more deeply pene- 

 trating part had made a most marked development. On July 5 its growth 

 had just well begun, but by September 2 it had made a really remarkable 

 development. The number of these more vertically penetrating roots varied 

 from about 20 to 35. They either ran straight downward from the base of 

 the plant or obliquely outward to a distance of 2 feet or more, and then with 

 a graceful curve took the perpendicular line of growth. Of these roots a 

 few were short and did not grow deeper than 1 or 2 feet; many reached a 

 depth of 7 feet, while still others were deeper. The maximum depth of pene- 

 tration was 8.2 feet. At the point of origin these roots were 2 to 5 mm. in 

 diameter. They were all profusely branched with laterals varying from less 

 than an inch to 1.3 feet in length. It was not unusual to find 10 or 12 of 

 these branches on an inch of the main root. The longer laterals were 

 rebranched. 



The period intervening between the second and final examination was 

 extremely dry and hot. During the whole interval of approximately two 

 months, the precipitation was only about 2.5 inches, which, with one excep- 

 tion, fell in showers so light as to be of very little value to the crop. The 

 exception was August 25, when the precipitation measured slightly over 0.75 

 inch. 70 per cent of the days were clear. The average daily temperature 

 for the period was about 82° F. and on several occasions temperatures of over 

 100° F. were recorded. Due in part to the low humidity accompanying 

 these conditions, the daily evaporation was high. Until July 22 it averaged 

 only 24 c. c, but for the next two weeks interval it reached an average of 

 41 c. c. daily. Because of these drought conditions the available soil-mois- 

 ture became greatly depleted. During the part of the period in July the 

 average available water-content for the first 3 feet was 8.7 per cent. On July 

 29 the fourth and fifth foot of soil had 12 per cent. During August water- 

 content was greatly reduced, the average for the first 3 feet being only 5.6 per 

 cent. On September 2, when the roots were finally excavated, the soil to a 

 depth of 5 feet had only 3.5 per cent of available moisture. The dry soil was 



