ROOT SYSTEMS OF SHORT-GRASS-PLAINS SPECIES. 43 



of these species is a reliable criterion of favorable or unfavorable 

 conditions for crop development. This is further discussed in Chap- 

 ter VI (cf. also Shantz, 1911, and Clements, 1920). 



Opuntia polyacantha. — This cactus enjoys a wide range from the subclimax 

 prairie along the Missouri, through the true and mixed prairies to the short- 

 grass plains of Colorado and New Mexico. In fact, it reaches its best develop- 

 ment and greatest abundance under the more arid conditions. It is greatly- 

 favored in its competition with the grasses by close grazing of the latter, and 

 consequently it becomes an indicator of overgrazed areas. In such places it 

 is frequently the most conspicuous feature of the landscape (plate 1 1 , a) , 



Three specimens growing on the short-grass plains were examined. As in 

 the case of Opuntia camanchica and 0. fragilis (Weaver, 1919: 61), and other 

 cacti (Preston, 1900), the root system consists of two distinct parts — a few 

 vertically descending anchorage and deep absorptive roots and a much more 

 extensive absorbing system in the surface soil. The deeper roots of the speci- 

 mens examined were only 3 to 5 in number, about 2 to 3 mm. in diameter, and 

 glistening white in color. They were fairly well branched, with rebranched 

 laterals mostly 0.2 to 0.3 foot in length, and reached depths of 1.5 to 2 feet. 

 Unlike the brownish surface roots, they were quite fragile. The surface root- 

 system consists of 14 to more than 24 roots, varying in diameter from 5 to 6 

 mm. to mere threads. They originate from the base of the plant and often 

 from the partially buried branches. These roots seldom penetrate beyond 3 

 or 4 inches and are often found nearer the surface. They pursue rather tortu- 

 ous courses parallel with the surface of the soil and reach a distance of 5.5 feet 

 from the base of the plant. For example, one root, 3 mm. in diameter and 

 5 feet in length, was traced at a depth of 3 inches to a distance of 4 feet from the 

 place of its origin. The roots taper very slowly in spite of much branching. 

 They branch repeatedly from their origin at the base of the plant to their 

 extremity, with both large and small branches, which ramify in all directions 

 and thus furnish an enormous absorbing surface. The ultimate root-ends, 

 whether of the shorter or larger branches, consist of much-divided and very 

 delicate brush-like termini, to which the soil clings with great tenacity. Only 

 the larger roots run for distances of several inches without a profuse supply 

 of laterals. As a whole the root system is strikingly like that of Opuntia 

 camanchica and is distinctly superficial in its position in the soil. Such plants 

 can compete successfully with the grasses for the moisture in the surface soil 

 afforded by summer showers. 



Schedonnardus paniculatus. — This annual or sometimes short-lived peren- 

 nial grass is nearly always present in small numbers in short-grass sod. On 

 denuded areas it becomes more abundant and usually replaces the earlier weed 

 stages and is in turn replaced by more stable vegetation. It is an excellent 

 indicator of overgrazing or other disturbance of the natural plant cover. It 

 was examined on the short-grass plains near Sterling, where some bunches 

 were growing in the open sod of buffalo grass and grama. The shallower 

 roots are very much like those of Aristida purpurea. They spread out later- 

 ally, at 0.1 to 0.2 foot below the surface, in such a manner that at a depth of 

 0.4 to 0.5 foot they may be from 1 to 1.5 feet horizontally away from the plant, 

 although some penetrate more or less vertically downward. These roots were 

 traced to a depth of 2 feet, where they broke off so easily and were so nearly 

 like those of Bouteloua gracilis that, although several clumps were examined, 

 none could be recovered in their entirety from the hard soil to a greater depth. 

 It is probable that they extend a little deeper, although the root system as a 

 whole is relatively shallow. 



