8 



THE ECOLOGICAL RELATIONS OF ROOTS. 



but the laterals are only 0.5 to 2 inches long. These often branch again. The 

 roots below the 18-inch level are relatively few and the soil is not well occupied. 

 Roots were examined in 3 different soil types and found to be very similar in all. 



Aristida oligantha. — This species is abundant on prairies, especially in drier 

 soils. It frequently dominates areas where the soil is shallow, or plays the 

 role of an interstitial among the taller grasses. The root systems of 10 plants 

 of this annual grass were examined. They were growing in a clay-loam soil. 

 The white, threadlike roots are less than 0.2 mm. in diameter and are found 

 abundantly only in the first 10 inches of soil. While some of the longest 

 reach depths of 40 inches, relatively few occur below 18 inches. They are 

 so fine and brittle that it proved impossible to secure material for a photo- 

 graph. Compared with other grasses the roots are much less abundant. 



Bulbilis dactyloides. — Perhaps no grass of the western plains, except 

 grama, is better known for its valuable characteristics as a pasture grass than 

 the buffalo grass. In the region of this study it can not compete with the 

 taller prairie plants. However, it is found in dense patches resulting from its 

 method of propagating by stolons, on low-lying lands where water may stand 

 for a time in ''pockets" or where over-grazing has killed the taller grasses. 



Ten Eyck (1904), working at Manhattan, Kansas, and in a soil of which the 

 surface foot was a mellow, dark loam, underlaid with a rather compact, clayey 

 subsoil, found "the roots are numerous but they do not penetrate deeply 

 into the soil". Shantz (1911: 38), dealing with the hard soils of the Great 

 Plains region of Colorado, where "the soil is only rarely wet down to a foot 

 or more," states that "almost the entire root system of the short grasses 

 {Bulbilis dactyloides and Bouteloua oligostachya) is limited to the first 18 inches 

 of soil. They have a very extensive surface root system." Robbins (1917: 

 70) states that roots of buffalo grass sometimes go to a depth of 7 feet. This 

 latter statement checks up more nearly with our findings. 



A dozen root systems of this grass were examined in two different areas, 

 both of which were in alluvial soil on bottom lands. The roots are very fine, 

 the largest being less than 0.5 mm. in diameter. They scarcely spread at all 

 laterally, but form a dense mat to a depth of 12 to 18 inches, branching mostly 

 only to the second order. These hairlike laterals are usually not over 0.5 

 inch in length. Although these roots are very fine, they are quite tough and 

 easy to follow. At 18 inches depth they become less numerous and are poorly 

 branched, so that the deeper soils are sparsely occupied. However, many 

 roots occurred at 4.5 feet, and numerous others continued vertically downward 

 to a maximmn depth of from 58 to 73 inches and in one of the trenches in 

 gumbo soil to the ground-water level. The shallower roots are light-brown 

 in color, while the deeper hairhke roots are nearly white. 



Bouteloua gracilis. — Grama grass occurs often in fairly pure stands on the 

 lighter soils of gravelly ridges and it is also frequently found dominating 

 areas of alluvial soil on bottom lands. Figure 1 shows a typical area domi- 

 nated by Bouteloua into which Andropogon furcatus is invading. The soil is a 

 very porous coarse sandy to gravelly loam deposited by glacial action, with a 

 water-holding capacity of about 40 per cent of its dry weight. This grama 

 consocies, in which a number of root systems were examined, dominates a 

 ridge at Belmont near Lincoln, which is entirely smTounded by tall prairie 

 grasses rooted in clay loam. The transition from one soil type to the other is 

 very abrupt and the ecotone is correspondingly sharp. Frequent determina- 

 tions invariably showed a higher available water-content in the clay-loam soil. 



The root system is extremely well developed, great masses of fine roots 

 occupying every cubic centimeter of soil to a depth of 18 inches. A few roots 



