12 



THE ECOLOGICAL RELATIONS OF ROOTS. 



had a root 7 mm. in diameter and a total length of 42 inches. The larger was 

 10 mm. in diameter and 68 inches deep. The laterals that do occur may- 

 originate near the surface and are branched to the third or fourth order. One 

 plant sent off two strong laterals at a depth of 12 inches. The deeper roots 

 are very fine and quite well branched. All have a characteristic yellow color 

 throughout. 



Vernonia baldwinii. — ^This weed propagates by strong rhizomes, a centi- 

 meter or more in diameter, which occur about 4 inches below the soil surface. 

 The roots, which are very abundant (20 or 30 arising from a single plant), 

 are very different from any other roots examined. They are tough, smooth, 

 yellowish-white, and unbranched throughout the first few feet of their course. 

 While the stele is only about 1 mm. in diameter, the fleshy cortex is thick 

 enough to give a total root diameter of from 3 to 9 mm. Many spread to a 

 distance of 20 inches from the base of the plant before turning downward. 

 Thus in a weedy pasture the surface soil is filled with the shiny, white, 

 unbranched, rope-like roots. At about 3 feet in depth they begin to branch 

 and the branches become more numerous in the deeper soils. Most of the 

 finer branches and terminal rootlets, however, occur at 9 or 10 feet. Seven 

 plants were examined, all of which penetrated to a depth of about 11 feet. 

 The maximum depth recorded was 11 feet 6 inches. 



Kuhnia glutinosa. — This prairie plant has deeper roots than most other 

 species examined. The maximum depths of 4 plants were 16 feet 8 inches, 16 

 feet 6 inches, 16 feet 10 inches, and 17 feet 3 inches, respectively. A part of 

 one of these is shown in plate 8, a, where only about a third of the root 

 system is exposed. The root in figure 3 was reconstructed from the photo- 

 graph of another root and a diagram made to scale in the field while excavating 

 the plant. This diagram shows all of the larger roots, the extent of their 

 lateral spread, degree of branching, etc. Although single water-content 

 determinations without wilting coefficients are usually of little value, yet here 

 the wetness and dryness of the various soil strata and the corresponding root 

 branching were so marked as to warrant including duplicate determinations 

 made on November 5, 1917, after several weeks of very dry weather. The 

 clay layers at 8 and 12 feet respectively are more or less impervious, and 

 collect and hold water readily. The tap-roots vary from 1.2 to 3.5 cm. in 

 diameter. All the roots are nearly white in color. 



Verbena stricta. — This weed, very common in old pastures, has a strong 

 tap-root from 1 to 2 cm. in diameter, which reaches depths ranging from 3.5 

 to over 4 feet. Seven plants were examined. In all cases profuse branching 

 occurred from the soil surface to the extreme root-tip. Because of the numer- 

 ous strong laterals sent off from the tap at all levels, the latter decreases in 

 size rapidly. These lateral branches lay hold of the soil to a distance of 18 

 inches on all sides of the plant. Small rootlets are abundant, the whole 

 forming an extremely well-developed root system. 



Grindelia squarrosa. — ^This weed has a strong tap-root abundantly supplied 

 with well-developed laterals. The main root varies from 0.5 to 1 inch in 

 diameter, but it gives off laterals so abundantly that this size is not long 

 maintained. Maximum depths of 50, 52, 40, and 73 inches were recorded for 

 the 4 plants examined. There are many very fine roots, 3 to 9 inches long, 

 occupying the surface soil. The larger laterals begin to branch off in the first 

 foot of soil, and run off obliquely from the main root to a distance of about 

 2 feet. Like the tap, these are supplied with abundant rootlets, the whole 

 forming a very extensive absorbing surface. The roots have a light yellow color. 



