ROOT SYSTEMS OF TRUE-PRAIRIE SPECIES. 27 



level is easily determined. Where unbranched or poorly branched 

 tap-roots are concerned, the working depth is often synonymous with 

 maximum depth. In almost every case the working depth has been 

 determined and averaged for several individuals of a species. 



Another important character of the root habit is also shown in the 

 table. This is the lateral spread of the roots in the surface soil. In 

 general this is not pronounced and is usually only 1 to 4 inches. It is 

 greatest among the shallow-rooted species composing the first group, 

 but even here some have few roots in the surface layer, while in most 

 cases the shallower roots run obliquely in such a manner as to reach 

 their maximum spread in deeper soil. Examination of the deeper- 

 rooted species shows that relatively few (only about one-fifth) rely to 

 any marked degree upon the shallow soil for their water and solutes, 

 while many carry on relatively little absorption in the first, second, or 

 third foot. This root habit is in marked contrast to that of the major- 

 ity of plants of the more arid mixed prairies and short-grass plains, 

 where surface rootlets are often extremely well developed. This root 

 habit is also very marked in sandhills vegetation, as has been pointed 

 out heretofore (Weaver, 1919). Further studies have added more 

 evidence to warrant this conclusion. Of all the grassland habitats 

 where water-content readings have been made, the prairies show rather 

 uniformly greater moisture in the deeper soils. This, it is believed, 

 accounts for the deeply penetrating root habit of most prairie species. 

 Plains species need to rely much more upon water supplied to the sur- 

 face soils by summer showers, and the response has been, in many 

 species, the development of widely-spreading shallow roots. Even 

 Bouteloua gracilis and Bulbilis dactyloides, grasses with notably wide- 

 spreading surface roots, under true-prairie environment at Lincoln 

 show only a small lateral spreading of their roots (cf. tables 1 and 9). 



COMPARISON OF TRUE- AND MIXED-PRAIRIE ENVIRONMENT. 



A comparison of the environmental conditions at a typical true- 

 prairie and plains station respectively (plate 4), together with an exact 

 picture of the vegetational covering as revealed by chart quadrats and 

 quadrat-bisects, goes far toward illustrating and explaining these varia- 

 tions in root habits. 



At Lincoln, Nebraska, in the true prairie, the mean annual precipita- 

 tion is 28.6 inches. At Colorado Springs, Colorado, in the mixed 

 prairie, it is only 14.6 inches. At both stations most of the precipita- 

 tion falls during the growing-season, a distribution of moisture very 

 favorable for the growth of grasses. Although the summer rains in 

 both areas are not infrequently torrential, the run-off at the mixed- 

 prairie station (as is true in general over most of the non-sandy plains) 

 is much greater because of the compactness of the soil. In short-grass 



