THE PLAINS ASSOCIATION. 



53 



distinctly shallow absorbing system. Many of the laterals run out to a dis- 

 tance of 12 to 14 inches and at a depth of only 1 to 4 inches, while below and 

 inside of these at all angles to the vertical they are very abundant. The 

 maximum depth for several roots was 6 feet. They are very abundant to a 

 depth of 3.0 feet. The surface absorbing roots were especially well branched. 



Stipa comata. — This grass often appears to be a dominant in the plains 

 association where overgrazing has not thrown the advantage in favor of the 

 short grasses. Although its aerial part is not extensive, it has a much deeper 

 and better developed root sj^stem than the corresponding species of the 

 prairies, >S. spartea. A trench 6 feet long and 5.5 feet deep was dug in a nearly 

 pure Stipa community. From the base of the clumps arise exceedingly 

 numerous fibrous roots, 1 mm. or less in diameter. While many of these 

 descend vertically, others run off at various oblique angles, some of them 

 having a lateral spread of more than 18 inches from the base of the plant, 

 while at a distance of 14 inches horizontally they reach a depth of 6 inches 

 (plate 19, a). Beginning at the very surface of the soil, the main roots are 

 clothed with relative!}^ short but well-branched laterals, 2 to 20 mm. long. 

 Thus the fii-st 28 inches of soil are thoroughly occupied by the main roots, 

 which are only a few millimeters apart, the interstices being completely 

 occupied by horizontal branches. Even to a depth of 32 inches the soil is 

 fairly well filled with much kinked and rebranched threadlike rootlets, while 

 not a few reach a maximum depth of over 5 feet. Thus Stipa comata is pro- 

 vided with a much finer, more branched, more widely spreading, and deeper 

 root system than is Stipa spartea of the prairies. 



Lithospermum linearifolium. — This is a common and rather conspicuous 

 plant of the grassland formation. Eastward it forms vernal societies. The 

 plant examined had a crow^n of 10 stems arising from the top of the tap-root, 

 which was 13 mm. in diameter. The tap proceeded straight downward 2.5 

 inches, where it divided into two equal branches, each about 7 mm. in diam- 

 eter. These branches then took a downward course, their diameter decreasing 

 very rapidly, so that at a distance of 22 inches from the surface they were not 

 more than 1.5 mm. wide. This diameter remained almost uniform for several 

 feet. They continued their downward couree almost vertically, one part 

 branching again at a distance of 3.5 feet and the other at a depth of 5 feet. 

 These branches of the second order also took a downward course, being not 

 more than 0.8 mm. in diameter. Lower down, one of these branches again 

 branched and penetrated to the maximum depth of 10 feet, ending in hairlike 

 termini. The root was very dark, almost black in color, and had a papery 

 cortex which could be removed very easily, revealiag a white hard and very 

 brittle stele. Throughout the lower 5.5 feet the roots were fairly well supplied 

 with short, attenuated branches 0.5 to 3 cm. long. Above this point shorter 

 branches were few in number. Yevy Httle absorption took place in the first 



4 feet of soil (fig. 14). 



Lygodesmia juncea. — Although this plant is of frequent occurrence, espe- 

 cially in drier situations, it plays a role of no great importance in the grass- 

 land. It is interesting because of its xeroid-shoot habit. Two plants were 

 examined. Each had a tap-root about 7 mm. in diameter, which took an 

 almost vertically downward course, in one plant penetrating to a depth of 



5 feet 8 inches. At 10 to 12 inches from the surface strong laterals were given 

 off, which (after taking a horizontal course from 8 to 22 inches) turned down- 

 ward not unhke the tap-root. Practically no small branches were given off 

 and only very seldom, as shown in figure 15, did any branching at all occur. 

 The roots throughout, both tap and laterals, pursued a very zigzag and tortu- 



