THE PLAINS ASSOCIATION. 



49 



face to the average working depth, and are abundantly supplied with short, 

 much rebranched laterals. Many of these spread very widely laterally, 

 branching and rebranching into both long and short termini, so that the 

 surface 27 inches of soil is completely filled with a dense network of absorbing 

 rootlets. Every cubic centimeter of the soil to a depth of 2 feet is occupied by 

 a network of this absorbing system. At a depth of 27 inches most of the root- 

 lets end in very much branched termini, but a few penetrate to greater dis- 

 tances. By undercutting the face of the trench and working upward from a 

 depth of 5 feet, several of these threadlike rootlets were encountered at a 

 depth of 55 inches. The roots are tan in color. 



Gutierrezia sarothrse. — ^This half-shrub occurs throughout the plains region, 

 although it is usually more abundant in areas of less thoroughly disintegrated 

 soil or in overgrazed areas. In such situations it forms extensive societies. 

 The plants examined were quite abundant and from 6 to 9 inches high. Nu- 

 merous stems arise from a strong tap-root from 5 to 8 mm. in diameter. The 

 tap immediately gives rise to 5 to 9 large laterals, 2 or 3 mm. in diameter, and 

 as a result, at a depth of 2 to 6 inches the tap is no larger than one of these 

 laterals. The main root descends more or less vertically and at a foot in depth 

 is seldom over 1 mm. in diameter. It is much curved and kinked, but these 

 twists are usually small, so that at a maximum depth of 6 or 6.5 feet the tip 

 of the root is not more than a foot or 18 inches horizontally from the base of 

 the stem. Just at the surface, to a depth of 12 to 18 inches, very numerous 

 threadlike laterals, 0.5 mm. or less in diameter, run off horizontally in all 

 directions for a distance of 3 to 24 inches, the plant thus being well supplied 

 with surface feeders. The large laterals frequently pursue a course nearly 

 parallel with the surface of the soil at a depth of 2 to 5 inches, or run some- 

 what obliquely to distances of 12 to 20 inches before turning rather abruptly 

 and vertically downward. Like the tap, they are profusely branched and 

 rebranched with short, threadlike laterals, but after a depth of 18 inches is 

 reached the branching becomes much poorer. These deeper roots may run a 

 distance of a foot or more without giving off any laterals, or again short laterals 

 only 2 or 3 inches long may occur in groups. The tips of these deeper roots 

 are only fairly well branched. Some were found which gave off a whole net- 

 work of branches matted in the cracks. While the larger roots are brown, 

 tough, and rather woody, the deeper ones are nearly white and rather brittle. 

 Several roots were traced to a depth of over 5 feet, while a few reached a 

 maximum depth of 6 to 6.5 feet (fig. 11). 



Psoralea tenuiflora. — ^This is one of the important legumes throughout the 

 plains. Under favorable conditions of soil moisture it forms extensive societies. 

 A number of individuals of this species were examined. They all had strong 

 tap-roots varying in diameter with the age of the plant. One large root, 

 which unfortunately was dead, had a diameter of 2 cm. At a depth of 1.5 feet 

 it gave off numerous large lateral branches, the branching system being not 

 unlike that of the more robust form of the prairies. A younger specimen, 

 with a tap-root 7 mm. in diameter, pursued an almost vertically downward 

 course to a depth of 42 inches. At a depth of 2 feet a small lateral ran off 

 rather horizontally to a distance of 20 inches. No other branching occurred 

 except near the tip, where 5 large branches and several smaller ones originated. 

 Except for a very few rootlets, 5 to 10 mm. long and distributed very irregularly 

 along the course of the tap, no other branching occurred. 



A large plant, with a tap 8 mm. in diameter, reached a depth of 8 feet. 

 Like the former, it pursued an almost vertically downward course. It was 

 unbranched to a depth of 28 inches, where it gave off a horizontal lateral only 



