14 



THE ECOLOGICAL RELATIONS OF ROOTS. 



level 3 other branches of equal or slightly smaller diameter (plate 9, a). They 

 ran off obliquely in 4 different directions, but none to a distance greater than 

 10 inches before they turned rather vertically downward; two more branches, 

 each 4 mm. in diameter, were given off at depths of 6 and 8 inches respectively. 

 All of these branches were very poorly supplied with laterals, as has already 

 been noted for the younger plants. They taper uniformly, so that at a depth 

 of 2 feet none had a diameter of more than 3 mm. The tap as well as several 

 of the ultimate branches of the laterals reached a depth of 6.5 feet. The 

 hairhke termini are sometimes unbranched, often for several inches, but are 

 more often well-branched. Nodules 1 mm. or less in diameter occur at all 

 depths, even near the root-tips. The roots are very brittle and hard to 

 recover from the joint clay. They are yellowish brown in color except the 

 older portions, which are dark brown. 



Psoralea tenuiflora. — This very abundant legume forms societies over large 

 areas of prairie during June. Indeed, for a period these tall, coarse herbs quite 

 overtop the grasses. The one shown in plate 10, a, had a tap-root 3 cm. in 

 diameter, which ran vertically downward. At a depth of 27 inches it appeared 

 dead, but it was traced to a depth of 52 inches, where it was still 3 mm. in 

 diameter. At a depth of from 1 to 2.5 feet many laterals occur. These are 

 from 7 to 12 mm. or less in diameter and frequently run off obhquely (as 

 shown in the figure) for distances of 8 to 18 inches before turning, often 

 abruptly, vertically downward. Several of these laterals, including some of 

 the smaller ones, reached depths of 5 or 6 feet. The lateral spread is such 

 that at the tips many of these roots are at a horizontal distance of from 18 

 to more than 24 inches from the base of the plant. 



Little absorption occurs in the first 2 feet of soil and this entirely from lateral 

 roots. The laterals are very poorly furnished with fine branches, although 

 at intervals of about a centimeter short branches occur not unlike those of 

 Astragalus. The root extremities, while very fine, are poorly branched. In 

 color the roots are reddish-brown, except the younger portions, which are 

 tan-colored. They are rather soft and more or less herbaceous, and have a 

 papery bark which is readily peeled off from the older ones. 



Psoralea argophylla. — This legume, which is much less conspicuous than 

 the preceding, forms estival societies over much of the prairie region. Plate 

 10, B, shows the dominant, httle-branched tap-roots of several plants of this 

 species. A single root will be described. This had a tap (v/ith a diameter of 

 8 mm.) which tapered so rapidly that at 2 feet it was less than 1 mm. wide, 

 and then pursued a vertically downward course to the water-level at a depth 

 of 6 feet. No large branches were given off except at a depth of 2 feet, where 

 a lateral occurred which was equal in diameter to the main root. This ran 

 quite parallel with the tap and at a distance of 4 to 6 inches from it, both 

 reaching approximately the same depth. 



Except for the surface foot, exceedingly fine branches, not more than an 

 inch in length and rarely rebranched, occurred at intervals of about 5 mm. 

 throughout the entire course of the root. At 3 inches from the tip the roots 

 each divided into 2 or 3 branches, aU of which were branched to the first order 

 only. The roots are dark brown to black in color. 



JBaptisia bracteata. — This showy legume, which forms vernal societies, has 

 very characteristic roots, the older portions being greenish-yellow and the 

 younger ones orange in color. From the base of the crown the roots spread 

 out obhquely and run downward, but in such a manner that the most widely 

 spreading were less than 18 to 24 inches horizontally away from the base of 

 the crown at any depth (plate 9, c). The main roots are poorly branched 



