THE PLAINS ASSOCIATION. 57 



4 feet square under the plant. They are orange-brown in color, the deep roots 

 especially being easily followed and distinguished from their competitors by 

 this character (fig. 17). 



Petalostemon candidus. — A group of more than 8 stems, all in blossom, 

 arose from a woody tap-root 17 mm. in diameter. At a depth of 3 inches this 

 tap split into three strong parts, 5, 6, and 8 mm. in diameter, respectively. A 

 larger branch ran obliquely for a distance of 3 inches and divided into two 

 equal parts, each 3 mm. in diameter. One of these descended rather verti- 

 cally, but with long, loose loops, 2 to 4 inches in width, turning back and 

 forth as was characteristic of other roots examined. The second fork ran off 

 obliquely about 4 inches further before turning downward. At a depth of 12 

 to 18 inches both branches divided and rebranched again and again, so that 

 at the 18-inch level none of the roots were more than 1.5 mm. in diameter. 

 Below the second foot the roots ran for long distances, following much curved 

 and tortuous courses but with little change in diameter. The branches that 

 were given off at frequent intervals were very long and not much branched. 

 Finally, in the soil from the third to the fifth foot the branches became more 

 numerous and almost microscopic, the root-tips being well supplied with 

 laterals only a few millimeters in length. 



The other two main branches were very similar to those described, and 

 spread in such a manner that a cross-section of the absorbing area under the 

 plant would not include more than 4 square feet. Several of the hairUke 

 ultimate rootlets reached a depth of over 5 feet; the deepest one examined, 

 that of the vertically descending tap, reached a maximum depth of over 5.5 

 feet. After examining about a dozen species of each color, it was found that 

 the purple prairie-clover had many more superficial branches, which lie nearer 

 the surface. The roots are yellower in color, the older ones being almost 

 black. The branches are fewer, seldom more than 3, and run downward at a 

 much sharper angle. The types described are very characteristic. Thus it 

 may be seen that the white prairie-clover, Petalostemon candidus, in com- 

 parison with the purple prairie-clover, Petalostemon purpureus, is not supplied 

 with absorbing roots in the surface 18 inches of soil, but gets the bulk of its 

 water and nutrients below this depth. The chief difficulty encountered in 

 excavating these plants was that of following the minute, dark-colored 

 termini in the third to the sixth foot of soil (plate 19, b). 



Eriogonum jamesii. — This species is widely distributed over the plains, 

 where it forms summer societies. The thick woody root, 3 cm. in diameter, 

 gave rise to a large number of prostrate stems which formed a mat about 8 

 inches square. Within the first 6 inches of soil, 10 or 12 laterals, the largest 

 of which was 5 mm. in diameter, ran off horizontally for a distance varying 

 from a few inches to 2 or 3 feet before turning downward. The tap tapered 

 gradually, so that at a depth of about 3 feet it was still 5 mm. in diameter. 

 To this depth it also gave off 2 other strong laterals and numerous smaller 

 ones, as in the surface 6 inches of soil. The former pursued a sinuous course, 

 such as is characteristic of Eriogonum roots, to a depth of several feet. At a 

 depth of about 3 feet, 2 other laterals arose which were only slightly smaller 

 than the tap. With the tap and rarely more than 12 to 18 inches from it, 

 these pursued an irregularly downward course, often turning backward and 

 forward horizontally or almost so, through a distance of 6 to 8 inches or more. 

 In the main, however, these roots as well as the tap had a vertically descending 

 direction. They were very poorly branched, rarely giving off small unbranched 

 wirelike laterals. The roots were traced to a depth of 7 feet 3 inches, where 

 they were still 2 or 3 mm. in diameter, respectively, and they undoubtedly 



