70 



THE ECOLOGICAL RELATIONS OF ROOTS. 



depths from the surface to 3 feet, the deeper ones undoubtedly having been 

 buried by wind-blown sand. From these rhizomes originate multitudes of 

 tough, wiry roots, 1 to 2 mm. or less in diameter, which penetrate the soil in all 

 directions from vertically downward to horizontally. These roots taper 

 gradually and many of them reach a maximum depth of 55 to 60 inches. 



From a depth of an inch to their very tips they are abundantly supplied 

 with laterals which extend out more or less at right angles to a distance of 1 or 

 2 inches or even more. These laterals are well provided with finer branches 

 to the third and fourth order. Since these roots arise not only from the surface 

 but also from the deeper rhizomes and penetrate to 3 or 4 feet in all directions, 

 they form a confused tangle. Where the grasses are at all thick, as shown in 

 plate 21, B, the roots completely fill the soil and bind it so thoroughly that a 

 vertical face of a trench 6 feet long and nearly as deep held tenaciously with- 

 out caving, even after it was somewhat undercut at the base. 



Andropogon hallii. — This tall, coarse grass forms loose, open bunches with 

 only a few large stems. It is a dominant along with A. scoparius in the bunch- 

 grass subclimax of the sandhill region. Successionally it occurs somewhat 

 later than the preceding sandhill grasses. It was excavated on a partially 

 captured sand-dune (plate 22, a). It is a very coarse, glaucous grass con- 

 nected by an elaborate system of rhizomes 3 to 4 mm. in diameter and lying 

 at a depth of 2 to 6 inches. From the rhizomes many roots take their origin, 

 some of which spread laterally, others penetrate almost vertically downward, 

 while still others take an oblique course downward. The diameter of these 

 varies from 1 to 2 mm. The roots running off laterally take a course almost 

 parallel with the soil surface or in some instances curve downward and then 

 up again to a point near the surface. These laterals lie at a depth of 2 to 16 

 inches and the maximum spread found was 3 feet. Of the roots taking a 

 vertical course the maximum depth found was 27 inches; most of them, 

 however, penetrated to a distance of about 22 inches or less. The oblique 

 roots were of about the same length, but since they pass off at an angle the 

 depth depends upon the degree of divergence from the vertical. All roots 

 throughout their course were copiously branched with mostly short branches 

 ranging from 0.5 to 3 inches in length; as many as 8 of these branches were 

 counted on a single inch of the root. These small branches were themselves 

 abundantly supplied with minute rootlets 1 to 5 mm. in length. Occasionally 

 the main roots gave off a branch 4 to 8 inches long, but these were very rare 

 and when they did occur were themselves branched in the same manner as 

 the main roots. Altogether, this plant has an excellent soil-binding and 

 absorbing system throughout the soil area of each bunch. Eight or nine 

 plants were examined. 



Muhlenbergia pungens. — This grass is characterized by tufted stems and 

 glaucous, narrow, rigid leaves. The culms, which arise from rootstocks, are 

 grouped in small tufts or cushions that lie close to the sand. It is a regular 

 component of blowout communities and is not infrequently dominant. 



This important sandhill grass was examined on the rim and grassy top of a 

 blowout where it was growing very abundantly. Clusters of roots arise from 

 the short rootstocks, which are 2 to 6 inches long and 1 to 3 mm. in diameter. 

 These vary in number according to the length of the rootstock, from only 4 

 or 5 to 25 or more (plate 22, b) ; the largest are only 1 mm. in diameter. While 

 some of the wirelike tough roots penetrate rather vertically downward to a 

 maximum depth of 33 inches, others run off obliquely at various angles, even 

 to almost parallel with the soil surface. The lateral spread varies from 10 

 to 25 inches on either side of the plant. Because of the dryness of the surface 



