ROOT SYSTEMS OF MIXED-PRAIRIE SPECIES. 



69 



water-content at depths of 5 and 6 feet was 9 and 10 per cent respectively. 

 Shantz (1917), in his study of plant succession on abandoned roads in eastern 

 Colorado, points out the effect which the destruction of the original plant- 

 cover produces on the water-content of the soil, the selection of the plant 

 populations in the various stages of succession, their reactions upon the habitat, 

 and the final replacement of such species as Stipa and Gutierrezia by a short- 

 grass turf. By absorbing the water in the surface soil, Bulbilis dacttjloides 

 and Bouteloua gracilis effectively cut off the moisture-supply to the deeper soils 

 and consequently deep root systems are no longer effective in supplying water 

 to the plant. The general stages of succession are similar at this station, but 

 the climax is mixed prairie. However, it must be kept in mind that Bouteloua, 

 and especially Bulbilis, while more superficially rooted than Stipa viridula, 

 have just as deeply seated absorbing organs as Gutierrezia sarothrce and many 

 other short-grass-plains and mixed-prairie species. It seems probable that the 

 cause of succession as regards a reduced water-content is due not entirely to 

 the position of the roots in the soil, but also to their efficiency in absorption. 



Table 5. 



-Water-content of soil in Stipa viridula consocies and in adjacent stabilized 

 mixed -prairie. 



Astragalus microlobus.— This legume forms estival societies which are es- 

 pecially well-developed in mixed prairie. Several plants were examined in the 

 sandy-loam soil near Limon. They are sometimes connected by rhizomes, 

 often only 4 to 8 inches in length and only 1 to 3 mm. in diameter. The tap- 

 root is well developed. One plant had a strong tap-root 10 mm. thick, which 

 descended rather vertically downward, tapering gradually, to a depth of 5.2 

 feet. A branch was given off at 8 inches in depth, while at 5 and 10 inches 2 

 laterals, each about 2 mm. in diameter, originated. These ran off rather 

 horizontally for a distance of 1 to 1.5 feet and then turned downward, reaching 

 depths of 3 and 5 feet respectively. As a whole, the root was poorly branched. 

 About a foot from its end the tap-root divided into several parts, each of which 

 branched repeatedly and in such a manner as to form a network of rootlets 

 several inches in extent. The ends of the large laterals terminated in a manner 

 similar to that of the tap-root. Aside from these, the branching was rather 

 poorly developed. The roots were considerably kinked and curved through 

 small distances. They are dark-brown in color. This rather coarse, moderately 

 branched root system, especially with its lack of surface absorbing rootlets, is 

 not greatly unlike that of Astragalus crassicarpus. 



Eriocoma cuspidata— This grass, although somewhat variable in habitat, 

 generally grows in sandy soil. It frequently plays an important part in re- 

 colonizing blowouts and is abundant also in more stabilized vegetation. 

 ^ A number of large bunches, growing at the head of a "break" near Ardmore, 

 South Dakota, were examined. Here the sandy and gravelly subsoil came 

 within a foot of the surface, where it was overlaid by Pierre clay. The plants 



