92 ROOT DEVELOPMENT IN THE GRASSLAND FORMATION. 



the evidence seems to point conclusively to nutrient or at least chem- 

 ical influences as the cause of variability in symmetry in the extension 

 of roots under dune conditions." In more arid regions, at least, the 

 evidence points to water-content as being the factor exerting the great- 

 est control, although nutrients undoubtedly play an important role. 



SUMMARY OF SHORT-GRASS-PLAINS AND MIXED-PRAIRIE SPECIES. 

 In order that a careful comparison may be made of the numerous 

 species excavated in short-grass plains, and in hard lands and sandhills 

 covered with mixed prairie, all of the species studied are listed in table 

 9, together with the extent of their root systems. 



CONCLUSIONS. 



An inspection of short-grass-plains species in table 9 shows that only 

 two, Opuntia polyacantha and Schedonnardus paniculatus, may be said 

 to be fairly shallow-rooted. All but one, Psoralea tenuiflora, are well 

 adapted for water absorption in the surface soil, having a lateral spread 

 of roots on all sides of the plant of about 0.5 to 1.5 feet. In this group 

 the three dominants have the best-developed root system in regard to 

 absorption both from the surface and deeper soils. 



Of the 45 species in table 9 which were examined in the mixed prairie 

 of sandhills, only four may be designated as shallow-rooted, but all of 

 these have a widely spreading root system. Among the 23 species 

 with roots of intermediate depth (which extended below 2 feet but 

 seldom beyond the 5-foot level), only 4 have a root system which does 

 not spread widely in the surface soil. In fact, the widely spreading 

 superficial root habit is a pronounced group characteristic. Of the 18 

 species of deep-rooted sandhill plants, among which are 3 dominant 

 grasses, all but 4 have widely spreading surface laterals in addition to 

 the extensive deep-seated ones. In Anogra cinerea, Asclepias arenaria, 

 and Psoralea lanceolata, the pronounced tap-root branches but little 

 or not at all in the surface soil, while in Pentstemon ambiguus the 

 numerous large branches are poorly fitted for surface absorption. 

 Summarizing the group as a whole, only 9 per cent of the species have 

 roots confined to the surface foot or two of soil, 18 per cent have few or 

 no roots which carry on absorption in this area, while 73 per cent of the 

 species are supplied with an absorbing system of such a character as to 

 get water and solutes from both the shallower and deeper soil layers, 

 many species having roots which extend as deep as the fifth to eighth 

 foot of soil. These data on root penetration, obtained from the study of 

 sandhills vegetation at 5 widely separated stations, emphasize the great 

 depth reached by many species and considerably modify the tentative 

 conclusions reached by the writer after studying the root development 

 in a single area (Weaver, 1919:68). It now appears that the sand- 

 hills area near Colorado Springs, where the earlier studies were made, 



