THE PLAINS ASSOCIATION. 63 



slightly more downward course, penetrating to a depth of not more than 8 

 inches. The first 3 or 4 inches of the root were sparingly branched, the few 

 branches being mostly mere tufts of rootlets not more than 3 or 4 mm. in 

 length. Throughout the rest of their course these divergent roots were 

 abundantly supplied with a system of laterals 0.2 to 0.3 mm. in diameter and 

 1 to 12 cm. long. These laterals were branched and rebranched to form 

 exteneive tufts interspersed throughout the soil, the whole forming a most 

 excellent system for surface absorption. Another plant gave a root depth of 

 15 inches. 



PLAINS ROOT SYSTEMS AND THE PLAINS ENVIRONMENT. 



An examination of the foregoing data impresses one with the great 

 depth to which the roots of most plains plants penetrate. There are 

 only 3 species of shallow-rooted plants that seldom extend below the 

 first 2 feet of soil {Kceleria cristata, Opuntia fragilis, and Ratihida 

 columnaris) . Perhaps Opuntia camanchica should also be placed here, 

 though its vertically descending roots reach depths of 3 feet. This 

 group constitutes only 11 per cent of the 28 plains species examined, 

 while in the prairies 6 grasses or 18 per cent of the 33 species ex- 

 cavated had shallow roots. In the second group, made up of plants 

 with roots extending well below the second foot of soil but seldom 

 deeper than 5 feet, are found 32 per cent of the plains species as against 

 27 per cent among prairie plants. The third group of deep-rooted 

 plants, which extend to depths greater than 5 feet, contains 57 per 

 cent of the plains plants and 55 per cent of prairie species. 



On the other hand, certain prairie plants, such as species of Rosa, 

 Lygodesmia, Liatris, and Kuhnia, reached depths of 17 to 22 feet. This 

 was quite beyond the greatest depth (about 13 feet) attained by any 

 plains plant. It may be that the soil is never moist to greater depths on 

 the plains. Moreover, most of the plains species are characterized, in 

 addition to their great depth, by a fine system of surface absorbing 

 and wide-spreading laterals. However, we must again resort to an 

 examination of the habitat factors in order to properly interpret the 

 root development. 



The region in which these plants grow has an annual precipitation 

 of only about 15 inches, an amount quite typical for much of the 

 Great Plains area. Fortunately, the major portion falls during the 

 growing season. There is as a rule considerable rainfall during the 

 months from May to September, but often the rainy season covers 

 only June, July, and August. The mean monthly and annual precip- 

 itation in inches for Colorado Springs, together with its seasonal dis- 

 tribution, is shown in table 14. 



Such a seasonal distribution of moisture is very favorable for the 

 growth of grasses. Because of the great compactness of the soil, brought 

 about in part by the binding influence of the short-grass cover, the run- 

 off is very high. Shantz has shown that at Akron, Colorado, the run-off 



