112 



THE ECOLOGICAL RELATIONS OF ROOTS. 



branching of the main roots, especially near their extremities) filled the soil 

 with a web-like network in marked contrast to the rather sparse, absorbing 

 rootlets of the shade form. 



Elymiis triticoides. — family of Elymus triticoides, which formed a sod 

 completely covering the ground under a canopy of Douglas fir, was examined. 

 The soil was fairly well decomposed and filled with humus to a depth of 18 

 inches. The underljdng soil was very similar to the second foot of soil in the 

 half-gravel-shde. A trench 4 feet long was dug through the midst of the com- 

 munity and careful examination showed that no roots penetrated to a depth 

 greater than 38 inches, while in the haK-gravel-slide a maximum penetration 

 of 46 inches by the larger roots was found (page 92). A marked difference was 

 evident both in the density and coarseness of roots, especially in the first 2 

 feet of soil. In the half-gravel-shde the roots were not only much coarser, but 

 they also filled the soil much more completely. In the shade form the flowering 

 staflis were also much less abundant. The ecads of this species show differ- 

 ences in depth of penetration and in the number of roots per given area, as 

 well as in size. 



Bouteloua gracilis. — This widely distributed and important grass has been 

 studied under four different habitat conditions. In the prairies of eastern 

 Nebraska it occurs often in fairly pure stands on the fighter soils of gravelly 

 ridges and is also frequently found dominating areas of alluvial soil on bottom 

 lands. One of the several plants obtained from a gravelly ridge at Belmont 

 is shown in plate 6, b, while the root system is fuUy described on page 8. A 

 few roots reached a maximum depth of 46 inches, although below 2 feet they 

 were very sparse. Great masses of roots occupied every cublic centimeter of 

 the soil to a depth of 18 inches. Other groups of plants examined in two 

 locations on alluvial soil showed a somewhat poorer development of the root 

 system, but the general distribution and depth were very similar to those 

 growing in the gravelly soil. 



This grass was again examined on the plains of Colorado, where Bouteloua 

 is the dominant, but where societies of Aristida purpurea, Psoralea tenuiflora, 

 and Artemisia frigida, with other deep-rooted plants, indicate rather favorable 

 water-content conditions. Here the soil was found to be weU filled with these 

 rootlets to a depth of 30 inches, while in the next 6 inches they were still fairly 

 abimdant, some of the longer ones penetrating to a maximum depth of 48 

 inches. The lateral spread in the surface soil was much more pronounced than 

 in the plants examined near Lincoln. The plains species have msmy wide- 

 spreading and exceedingly well-branched roots filhng the surface soil and 

 extending to 1.5 feet or more on all sides of the clump. 



This grass was likewise studied in the typical hard-land or short-grass 

 country about 25 miles southeast of Colorado Springs (p. 65, plate 20, a), 

 where Bouteloua again dominates and the shallow-rooted Opuntia polycantha 

 is the principal species, a community conspicuous for the almost entire 

 absence of Psoralea, Gutierrezia, Chrysopsis, and other deep-rooted dicotyle- 

 dons. A number of roots were found to penetrate to a depth of 51 inches, 

 v/hile they were very abundant to a depth of 40 inches. Their wide-spreading 

 habit and their position in the soil was almost identical with those ui the plains 

 habitat just described. 



Finally, a fourth group of these plants was excavated in the sandhills 

 (p. 68), about 40 miles southeast of Colorado Springs but under the same 

 general condition of rainfall. A trench 8 feet long and 5 feet deep was dug on 

 a well-covered sandhill where this species was the dominant and formed 

 rather pure stands. From the short, tough rhizomes, usually only an inch or 



