38 



THE ECOLOGICAL RELATIONS OF ROOTS. 



Hieracium scouleri. — Thishawkweed is one of the few species of the western 

 prairies which forms autumnal societies. For the 26 root systems examined 

 an average depth of 5.5 feet was determined. It sends out numerous roots, 

 as many as 50 from a single inch of its rhizome. The rhizome is 8 to 12 inches 

 long. None of these roots are over 3 mm. in diameter. They pursue a nearly 

 vertically downward course and throw off practically no laterals, except where 

 they enter earthworm holes. Here strong laterals, equahng the main root in 

 diameter, are developed and run parallel with it in the hole a foot or two or 

 further. All give rise to abundant root-hairs and it is seldom that these roots 

 again enter the soil. The tip of the main root may branch very profusely, or 

 very little if in hard soil. The longest root reached a depth of 7 feet 9 inches, 

 although 5 feet 4 inches was found to be the average depth (fig. 6) . 



Potentilla blaschkeana. — The tufted stems of this subdominant are borne 

 on a short, thick crown from which 2 to 5 main roots originate. These average 

 about 7 mm. in diameter. They taper off gradually till at about 2 feet in 

 depth they are often only 2 mm. in diameter. Here they usually branch 

 dichotomously, and again branching, break up into numerous small roots — 

 mere hairs — ^which have a strong tendency to grow in earthworm holes. 

 These they follow for perhaps 2 feet before entering the soil again. In these 

 holes they give off many threadlike branches which follow down the same 

 hole, branching profusely and often not reentering the soil. The roots are 

 dark brown in color. They are unbranched or very poorly branched at the tip. 

 Thirty root systems were examined. The longest roots penetrated to a depth 

 of 7.5 feet, while the average root depth was found to be about 5 feet (fig. 6). 



Eriogonum heracleoides. — This mat-former, at home on dry ridges, has a 

 strong, woody tap-root often an inch in diameter. It sometimes breaks up 

 just below the soil surface into several more or less equal parts. The roots 

 penetrate to a depth of 6 to 8 feet or more. They spread widely and are well 

 branched. 



Some examinations were also made of the roots of Rosa nutkana and Sym- 

 phoricarpos racemosus, both very common in nearly all prairie situations. 

 They were found extending to depths of from 6 to over 8 feet. 7ns mis- 

 souriensis roots penetrated the soils of dry hillsides to 3 feet 10 inches, and 

 Berheris repens to a depth of over 10 feet. 



PRAIRIE ROOT SYSTEMS AND PRAIRIE ENVIRONMENT. 



From these data it may readily be seen that most of the prairie 

 plants are deep-rooted. With the exception of Kceleria cristata, Poa 

 sandbergii, and Festuca ovina (which are confined largely to the surface 

 18 inches of soil) all of the species examined reached depths of from 4 

 to 6 feet, while some penetrated even deeper. However, when com- 

 pared with the species of the eastern prairie, these plants are not as 

 deeply rooted. It may be recalled (p. 18) that about 55 per cent of 

 the roots in the latter conununity penetrated well beyond 5 feet and 

 many of them beyond 10 or 12 feet. Here, however, with a few 

 exceptions, the plants usually terminate their root systems at depths 

 of about 5 or 6 feet, while only one or two species have roots penetrating 

 to 10 feet. A proper interpretation of this difference can be made only 

 by a careful study of the habitat. 



