78 



THE ECOLOGICAL RELATIONS OF ROOTS. 



largest examined being 4 mm. in diameter. This tap penetrates almost 

 vertically downward. Beginning at a depth of 2 to 3 inches, it sends off 

 multitudes of fine fibrous roots which are much branched and penetrate the 

 soil on all sides to a distance from a centimeter or less to 3 or 4 inches. In 

 addition to these finer roots, the tap sends off longer laterals which traverse 

 the soil in all directions, some taking a course almost parallel with the sur- 

 face, while others pass off more obliquely. These vary in length, some being 

 10 to 12 inches long. The laterals are much branched and rebranched, and 

 together with the finer rootlets form an effective absorbing system. Usually 

 at a depth of 5 to 8 inches the tap divides up into such a system of large 

 laterals as to almost if not quite lose its identity. These lower laterals vary 

 from 0.5 to 1 mm. in diameter and diverge more or less obliquely downward, 

 some penetrating to a depth of 26 inches or more. The deeper penetrating 

 divisions of the tap branch rather freely and end in much divided termini. 

 On the whole, however, these lower branches are more sparsely supplied with 

 absorbing rootlets than those nearer the surface. The roots are of a light 

 brown to white color, are rather fragile, and hard to trace. Six plants of 

 varying sizes were examined. 



Psoralea lanceolata. — This legume sometimes replaces Redfieldia flexuosa 

 as the pioneer in the shifting soils of blowouts, but normally it belongs later 

 in the succession. Its underground parts eminently fit it to succeed in a 

 habitat where the ground-line is constantly lowered by wind erosion or built 

 up by the same agency. 



The individual plants, which may be a few inches or several feet apart, are 

 connected by a more or less horizontal system of rhizomes, which varies in 

 depth from 2 or 3 inches to more than 2 feet. These connecting parts vary 

 from a few millimeters to more than a centimeter in diameter and may run 

 horizontally for distances of 10 to 30 feet or more. At irregular intervals 

 along their course occur erect portions, often a centimeter in diameter and 

 frequently forked at the top. These give rise to the individual plants, while at 

 other intervals, and not always below the erect stems, strong, rather vertically 

 descending roots occur, many of which reach a depth of 8 or 9 feet (fig. 29). 

 The surface 2 or 3 feet of the root system is poorly supplied with absorbing 

 rootlets. In fact, only a few of the major branches have their origin in this 

 layer of soil and the ultimate rootlets of these extend much deeper. The main 

 system of branches begins in the fourth foot of soil and extends to the 8 or 

 9-foot level. This consists of both large and small wide-spreading and well- 

 branched rootlets, the laterals often running out to a distance of 2 feet or more 

 from the base of the tap. Large root nodules, 1 to 3 mm. in diameter, were 

 observed at a depth of 8 feet. One large tap-root was traced to a depth of 

 over 9 feet, where it still maintained a diameter of 4 mm. It was not followed 

 further, because of the danger of caving the sandy-walled trench. The roots 

 were dark brown in color and rather tough. 



Ipomoea leptophylla. — The bush morning-glory is a common plant in the 

 sandhills, where the large hemispherical tops, conspicuous because of their pro- 

 fuse purple blossoms, cover many square feet. Two plants which were growing 

 together were examined. The top measured 7 feet in diameter and was 30 

 inches high. The multicipital stem arose from strong tap-roots 2.5 inches in 

 diameter. At a depth of 9 inches the taps became greatly enlarged, reaching 

 a diameter of 6 to 8 inches, which they maintained to a distance of about a 

 foot, when they tapered off gradually, so that at a depth of 4 feet they were 

 but an inch or two in diameter. Below this point the tap lost its dominance, 

 breaking up into large numbers of deeply penetrating and widely spreading 

 branches, as shown in figure 30. 



