68 ROOT DEVELOPMENT IN THE GRASSLAND FORMATION. 



In plate 14, b, may be found a specimen of Bouteloua curtipendula excavated 

 and photographed on August 25, 1919, and only 124 days after the seed was 

 planted in the greenhouse on April 23, the seedlings having been transplanted 

 in the field on May 10. Several fine specimens were thus raised on tilled soil 

 adjoining the high-prairie station at Lincoln, Nebraska (cf. pp. 28 and 126). 

 The plants were 2 feet tall and the clumps from 1 to 1.5 inches in diameter 

 at the base. The maximum root depth was 3 feet, while the maximum lateral 

 spread of 1.5 feet from the base of the clump was reached at a depth of about 

 2 feet. This conforms with the general root habit of prairie grasses, which as 

 a group do not show so great a tendency to spread widely in the surface soil 

 as do most species in the drier mixed prairies and short-grass plains. 



Further examination of this species was made in the typical mixed prairies 

 at the United States dry-land experiment station near Ardmore. The plants 

 were growing in Pierre clay, a soil of very hard texture, underlaid at a depth of 

 2.3 feet with a sandy and somewhat gravelly subsoil. The roots had the wide 

 lateral surface spread already described for those at Colorado Springs. The 

 working depth was found to be 3.7 feet, while some reached a maximum depth 

 of 4.3 feet. Near the root-ends the lateral branches were 2.5 inches long and 

 the ultimate rootlets were exceedingly fine and well developed. Thus Boute- 

 loua curtipendula is characterized by a widely spreading, moderately deep, 

 but exceedingly well-branched root system. 



Stipa viridula. — This species of porcupine grass is distinctly less xerophilous 

 than Stipa comata. It is usually best developed in soils which permit of con- 

 siderable water penetration and occurs in greatest abundance in the plains 

 region where the stable vegetation has been disturbed, and in broad swales. 



A number of specimens were examined in an old road near the base station 

 at Colorado Springs. It formed an almost pure luxuriant growth, being over 

 2 feet high (plate 22, c). The larger roots are coarse, tough, and wiry, espe- 

 cially in the first 5 to 7 feet of soil. They are about a millimeter in diameter. 

 Just below the soil surface some of the roots run off in a course almost parallel 

 with it and at a depth of only 2 to 4 inches to distances of from 1 to 1.5 feet or 

 sometimes even further before turning downward. Others run off obliquely 

 with a maximum spread of only 8 inches on either side of the plant. The rest 

 run rather vertically downward, some to a depth of over 11 feet. In the first 

 5 feet of soil the roots are much less brittle than in the deeper strata. For this 

 reason only a portion of the very extensive root system is shown in plate 16, a. 

 The root branching is very much like that of Stipa comata, the entire area 

 under the plant and for a distance of more than a foot on either side being 

 filled with the profuse and delicate branches of this very deep root system. 

 At a depth of 5 feet some of the roots are still a millimeter in diameter. The 

 lateral branches may exceed 3 inches in length and are well supplied with 

 delicate rootlets. Below 9 feet the roots are much finer. Some penetrated to 

 a maximum depth of 11.7 feet. Indeed, they are fairly abundant to 11 feet. 

 They end in well-branched tips, the branches being usually less than 1 inch in 

 length but poorly rebranched. The lateral spread of the roots of this species 

 is quite characteristic for grasses of the more xerophytic portions of the grass- 

 land formation, but the great depth of penetration is remarkable, even exceed- 

 ing that of Bulbilis dactyloides. 



The marked differences in the water-content of the soil in the abandoned road 

 among the roots of Stipa and in the undisturbed mixed prairie at the base 

 station about 15 rods away is shown in table 5. 



Even at depths of 5 to 12 feet the soil was quite moist (9.5 to 11 per cent 

 water-content) when the roots were excavated on June 23. On August 9 the 



