52 Development and Activities of Roots of Crop Plants. 



shown a profound development of the root system. The working level had 

 reached a depth of 2.8 to 3.1 feet; the longest roots penetrated the soil rather 

 vertically downward for 4.8 feet; the lateral spread had increased to a maxi- 

 mmn of about 1 foot, while a vast network of rebranched laterals occupied an 

 area of soil extending roughly 0.8 foot on all sides of the plant and to a depth 

 of 2 or 3 feet (fig. 21 b). The total number of roots had now increased to 

 20 or 25, varying with the number of tillers on a parent plant. Many of 

 these were more superficially placed than in earlier stages of development, 

 running off rather horizontally or obliquely and ending in the first 3 to 8 

 inches of soil. In the surface 2 feet especially, laterals were exceedingly 

 abundant, usually 5 to 9 occurring on an inch of root-length. Many of these 

 were short and few exceeded 3.5 or 4 inches in length. Secondary laterals 

 were not at all abundant. In the second foot the branches were mostly less 

 than an inch in length. Below 2 feet branching was somewhat less pro- 

 nounced, especially as regards length of primary rootlets and abundance of 

 secondary ones. In the fourth and fifth foot numerous roots were charac- 

 terized by unbranched laterals about 0.3 inch in length, but only 1 to 4 oc- 

 curred on an inch of root. On others the branches were much thicker but 

 shorter, and frequently occurred to within an inch of the root-ends. Again, 

 other root-ends were without branches for several inches back from their tips. 



In the lower plats the wheat at this time was 2.6 feet tall and blossoming 

 had just begun. As with the other cereals, tillering was much more pro- 

 nounced (average of 3.2 stems per plant as compared with only 1.7 in the 

 upland) and the stand was thick. The maximum root penetration was 4.8 

 feet and the working depth about 3.1 feet. No marked differences were 

 noted in the branching habit or extent of lateral spread. 



A final examination a few days after harvesting the grain on July 15 

 showed no great change in root development. As with oats and barley, the 

 roots, except the deepest ones, were somewhat shriveled and more brittle 

 than before. The depth and lateral spread had increased only slightly. In 

 the lower crop plats the working level had deepened to 3.5 feet, but no roots 

 were found below the former 4.8-foot level. As a whole, the root system 

 was a little finer and somewhat more extensive than that of oats. 



Alfalfa, Medicago sativa. 



This crop was sowed on the upland plats on March 31, in an area adjoining 

 the cereals. The environmental conditions until May 1, when the initial 

 examination of root development was made, have already been described 

 (p. 42). At this time the plants were not over 0.5 inch in height. The first 

 pair of true leaves was just unfolding. The tap-roots had penetrated to 

 depths of 5 or 6 inches and the first laterals were just beginning to form on 

 some plants. 



A second examination was made on June 2. The plants averaged only 

 2.5 inches in height, although certain individuals exceeded this by 0.5 to 1.5 

 inches. The stage of development of both tops and roots is shown in figure 

 24 A. Several roots reached depths of 1.5 feet, while two of the plants exam- 

 ined had roots 1.8 feet long. Practically no branches occurred in the surface 

 1 or 1.5 inches of soil, but below this level as many as 10 or 12 per hnear 

 inch of tap arose. These varied from a few millimeters to 3 inches in length; 



