114 



THE ECOLOGICAL RELATIONS OF ROOTS. 



Yucca glauca. — The root systems of numerous individuals of this species 

 were examined in the plains soil (page 51) and also in the half-gravel-slide of 

 the mountains. The main roots, which are more abundant in the haK-gravel, 

 slide ecad, are much more profusely branched. Great clusters of long, well- 

 branched sublaterals fill the soil in sharp contrast to the poorly branched 

 laterals characteristic of the half-dozen yuccas examined on the plains (fig. 

 52). A marked difference was also noted in the diameter and the fleshiness 

 of the system of roots. While most of those of the plains were rather dry and 

 often papery, those of the gravel-slide were generally more turgid, thicker, 

 and fleshy. The rhizome system is somewhat more branched and is nearer 

 the surface than on the plains. 



Fig. 52. — Fragments of the roots of Yucca glauca: a, plains form; &, half-gravel-slide form. 



Allionia linearis. — This four-o'clock is a widely scattered secondary species 

 on the plains and also occurs rather abundantly in the sandhills. Of the 8 or 

 more plants examined in the former situation, all had strong and rather 

 fleshy tap-roots, the largest 12 mm. in diameter. However, the tap is not 

 uniform, often contracting at the top and being largest at a depth of from 8 to 

 16 inches. At about 2 feet in depth it narrows down abruptly to scarcely more 

 than 2 mm. Except for small curves and kinks, it pursues a vertically down- 

 ward course, some to a maximum distance of 5.5 feet. The first 18 to 24 

 inches of the root are practically unbranched, and in fact the lower part of the 

 root system has no long laterals. Those that do occur at irregular intervals 

 consist of groups of hairlike laterals, seldom more than a centimeter in length 

 and usually only 4 or 5 mm. long. These are very minute in diameter and 

 rather well branched. The tip ends abruptly with only a few short branches. 

 The root is dark brown in color and very brittle, the interior parts being 

 glistening white. At a depth varying from 6 to 14 inches, usually one or 

 sometimes two laterals arise. These may be as large as 2 mm. and run off 

 horizontally or shghtly upward, ending 18 inches to more than 2 feet from 

 the base of the plant. Like the main root, they are only poorly clothed with 

 clusters of short branches (fig. 53). 



Other specimens were examined in the sandhills from a half-captured 

 blowout. Five roots were examined. In general they were very similar, the 

 strong taps (which were about 5 to 8 mm. in diameter) descended rather 

 vertically to a depth of 4 or 5 feet. A typical specimen is shown in figure 54. 

 One to three branches, usually only one, occurred in the first foot of soil. The 

 lower portions of the roots had a few branches or were in some cases almost 

 destitute of them. The tops were both greater in diameter and taller than the 

 plains specimens examined. The leaf surface also was greater. 



