4 THE ECOLOGICAL RELATIONS OF ROOTS. 



I. THE PRAIRIES.^ 



The prairies of eastern Nebraska in which these studies were made 

 are too well known to need discussion here (plate 2). A general floristic 

 description has been given by Pound and Clements (1898, 1900), a 

 more detailed one for a portion of southeastern Nebraska by Thornber 

 (1901), and recently a more specific account of the local region under 

 consideration by Weaver and Theil (1917). As the various species are 

 considered, brief comment will be made upon their relative importance. 

 Therefore, we may proceed to a consideration of the root habits of the 

 individual species and then examine the environmental conditions 

 under which the plants grow. Finally, the correlations between root 

 habit and environment will be discussed. This sequence will be fol- 

 lowed for each plant community. 



Panicum virgatum. — Although showing a preference for loose sandy soils, 

 panic grass grows abundantly in many situations throughout the prairies. 

 Its size, abundance, and duration combine to make it an important com- 

 ponent of the prairie vegetation, where it often holds the rank of a dominant 

 species. 



This grass has the longest root system of any species examined. The roots 

 are very coarse, many having a diameter of 3 or 4 mm. They pursue a verti- 

 cally downward course, spreading only a Httle near the surface, to a maximum 

 depth of over 9 feet. Several plants reached depths of over 8.5 feet (plate 

 4, a) . In the first 6 or 7 feet of soil the roots are very little branched, the 

 laterals (usually less than 3 inches long) occurring only scatteringly. In the 

 deeper soil numerous fine laterals occur, although the last 6 to 12 inches are 

 often very poorly branched and the roots end abruptly. The main roots 

 remain nearly uniform in diameter for most of their course or taper so slowly 

 that at 5 or 6 feet they may still have a diameter of 2 mm. The type of soil 

 considerably modifies not only the root penetration but also the manner of 

 branching. The plants that were growing in a gravelly soil with a sandy 

 subsoil underlaid with an impervious blue clay were found to penetrate only 

 7 feet to the clay, but the branching was much more pronounced. The cortex 

 on younger roots is pearly white and very brittle; on older ones smooth, 

 pinkish white, and of a papery consistency. Eight plants were examined. 



Andropogon furcatus. — The two bluestems are dominants among prairie 

 grasses. They form a large part of the prairie hay of Kansas and Nebraska. 

 The taller and deeper but cosirser-rooted A. furcatus is less resistant to drought 

 than the shorter, finer-rooted A. scoparius, as is shown both by their local 

 and general distribution. Throughout the prairies the former luxuriates in 

 the draws and on lower lands, while the latter dominates higher areas. West- 

 ward the big bluestem soon drops out, while little bluestem not only forms a 

 " dominant of widest distribution and most controlling influence in the bunch- 

 grass association of the sandhills" (Pool, 1914:224) and "enters into dis- 

 turbed areas of the wiregrass association in Colorado" (Shantz, 1911:52), 

 but also occurs on rough hillsides where water penetrates readily, throughout 

 much of the area included in Wyoming and Montana. 



^ The major portion of the work in this section was done by the writer in conjunction with Mr. 

 Lyman H. Andrews, who voluntarily joined the colors in December 1917, and to whom joint 

 credit is herewith acknowledged. 



