18 ROOT DEVELOPMENT IN THE GRASSLAND FORMATION. 



maintained their original diameter, while others tapered to an end at about 

 this level. In general, the branching is somewhat more pronounced below 

 the fifth foot, thread-like laterals 1 to 3 inches in length coming off in small 

 clusters. Of the four plants examined, the working level was determined at 

 about 5.5 feet, although many roots penetrated beyond. Some extended well 

 into the seventh foot of soil. The maximum depth recorded was 8.2 feet. 

 This species is similar to the preceding in penetrating deeply and relying but 

 little upon the surface soil layers for water and solutes. 



Agropyrum glaucum. — Wheat-grass is a dominant of both true and mixed 

 prairie vegetation. Because of its height and dense growth, due to its excel- 

 lent system of rhizomes, it frequently dominates local areas, sometimes almost 

 excluding other plants (plate 12, b). It forms extensive consociations in both 

 types of prairie vegetation, these usually being more extensive in mixed prairie 

 (plate 3, a, b). If at all abundant it is an indicator of favorable deep soil- 

 moisture conditions, but in thin stands and as dwarfed individuals it occurs in 

 rather dry soil. Height-growth seems closely related to soil-moisture. 



The plants here described were growing in the high prairie not far from the 

 base station. They were in complete control of the local area. The individ- 

 uals are connected by tough rhizomes from 1 to 2 mm. in diameter and from 

 0.3 to over 1.5 feet in length. The rhizomes, which lie 0.5 to 3 inches below 

 the surface, are frequently branched, abundantly covered with scale leaves, 

 and tipped with long, sharp-pointed buds. From the nodes of the rhizomes 

 at intervals of 1 to 1.5 inches, arise tough white roots. There are usually 

 2 to 4 at each node. Like those arising from the base of the clumps, they are 

 about 0.5 to 1.5 mm. in diameter, quite coarse in texture, and do not spread 

 much laterally from their place of origin. In the main their course is rather 

 vertically downward (fig. 3). They all branch profusely and somewhat reg- 

 ularly. The branches are usually less than an inch in length and often pur- 

 sue a course more or less at right angles with the larger roots from which they 

 arise. Some, however, are occasionally 2 to 3 inches long and run off obliquely 

 downward. The laterals throughout are very abundant to within 1 to 1.5 

 feet of the tip, below which point they are less pronounced. In fact, some of 

 the root-ends may extend for distances of 0.7 to 1.2 feet with scarcely any 

 branching, while on others, where laterals are present, they are usually short. 

 Many of the roots penetrate the moist soil to the working depth of 8 feet, while 

 the maximum root depth was about 9 feet. Specimens examined in low 

 prairie adjoining the base station (p. 37) showed, in this moister, richer, 

 alluvial soil, a working depth of 7 feet and a maximum penetration of 8 feet. 



This root habit is in rather close agreement with that described for the same 

 species growing in the mixed prairie near Colorado Springs. The chief differ- 

 ence is the surface absorbing system, which is much more poorly developed. 

 In the mixed prairie numerous short horizontal roots arose from the base of 

 the clumps and from the rhizomes. These were profusely branched and 

 rebranched to the third and fourth order, the ultimate branches being almost 

 microscopic in size and furnishing a splendid surface absorbing system (cf. 

 Weaver, 1919:52). This difference is a characteristic one for many species 

 of mixed prairie and short-grass plains as contrasted with those of the true 

 prairie. It is undoubtedly a response to the water-content. Moreover, the 

 root system was not so deeply placed in the more arid region, for few roots 

 were found below 7 feet, while the working depth was about 6 feet. 



Poa pratensis. — Kentucky bluegrass was probably formerly indigenous 

 only in the North and West. Since the settlement of the prairie region it has 



