46 THE ECOLOGICAL RELATIONS OF ROOTS. 



IV. THE PLAINS ASSOCIATION. 



This great grassland community extends from northwestern Texas 

 and northern New Mexico to northern Colorado and Nebraska and 

 from the Rocky Mountains eastward to central Texas and Kansas, 

 where it meets the prairie. While soil moisture seems to be the limit- 

 ing factor in the extension of the prairies westward, this is so inti- 

 mately connected with the amount and distribution of the precipita- 

 tion and with the soil type that great tongues of true prairie extend 

 far into the Great Plains. This is especially true northward, while over 

 much of the broad ecotone plains and prairie vegetation alternate. 

 The plains community differs from the prairie chiefly in the domi- 

 nance of short grasses, especially Bouteloua gracilis and Bulbilis dady- 

 loides, and in the smaller number and reduced importance of the societies. 



An excellent description of various plains communities, together 

 with their successional relations and indicator values, especially for 

 Colorado, has been given by Shantz (1911). Plate 18, a, affords a 

 ghmpse of the plains vegetation near Colorado Springs, Colorado, 

 where the roots described below were excavated. Here about 80 to 

 85 per cent of the soil surface was covered with vegetation. 



Bouteloua gracilis. — This well-known and important grass is a dominant 

 over the plains from western Nebraska to the Rocky Mountains and from 

 Texas far into Canada. From the standpoint of grazing it ranks among the 

 highest of all the grasses, being equaled only by Bulbilis dadyloides. 



The soil is well filled with fine rootlets to a depth of 30 inches, while in the 

 next 6 inches they are still fairly abundant, some of the longer ones penetrating 

 to a maximum depth of 48 inches. The surface roots spread very widely in 

 the shallow soil to 1.5 feet or more and are exceedingly well branched. 



Aristida purpurea. — Much of the plains association is characterized by the 

 bunches of this very widely distributed dominant, wherever overgrazing or 

 other disturbance has given it a foothold in competition with Bouteloua gracilis. 

 From the bases of the individual clumps strong fibrous roots arise, from 0.5 to 

 1 mm. in diameter. These rather coarse roots either descend vertically or run 

 off obliquely at an angle of from 20 to more than 45 degrees with the surface 

 of the soil to a distance of 5 to 8 inches before turning downward. The sur- 

 face of the soil below the clump is completely filled with these cord4ike roots, 

 but they do not branch profusely until they have penetrated 4 or 5 inches into 

 the soil. Below this depth for 3 or 4 feet the roots become smaller, giving off 

 both large and small laterals, many of the latter being only 1 to 10 mm. in 

 length. The ground is rather well occupied to a depth of 3 feet, at which 

 depth many of the roots terminate in rather well-branched tips, while others 

 penetrate to a depth of 4 feet or slightly beyond. A maximum depth of 4 feet 

 3 inches was recorded for several of these tiny, hairlike termini. Branching is 

 especially well developed in joints between the lumps of hard soil. The older 

 roots are ashy-gray in color, and are provided with a rather papery cortex, 

 which upon removal reveals the tough, light-yellow stele. The deeper roots 

 are threadhke and are very fragile and usually well branched, although they 

 sometimes run for several inches without giving off any laterals (fig. 9). 

 Fissures occur in the soil, extending to a depth of 2 or 3 feet. These have 



