78 



ROOT DEVELOPMENT IN THE GRASSLAND FORMATION. 



only 1.2 feet tall and was not thriving, because of the keen competition with 

 Bulbilis in the compacted silt-loam at the edge of a swale (plate 11, b). Here 

 the working depth was only 4.3 feet and the maximum root penetration 5.2 

 feet. A correlation between the height of the tops and the root depth in the 

 several communities (shown in table 8) is of particular significance, because 

 similar general correlations have been determined for the root systems of 

 crop plants. 



Table 8. — Development of Agropyrum glaucum. 



Gutierrezia saroihrze. — This perennial half-shrub forms societies throughout 

 the plains region. In areas which have been greatly over grazed, or where the 

 soil is rather poorly disintegrated, it often occurs in great abundance. 



The plants examined in the Bulbilis-Bouteloua turf at Sterling, like those of 

 Arisiida purpurea and Artemisia frigida (pp. 42 and 83), were dwarfed speci- 

 mens only 5 or 6 inches high. The numerous stems forming this woody half- 

 shrub arose from tap-roots 4 to 8 mm. in diameter. Just beneath the soil 

 surface the tap-root invariably gave rise to so many laterals (the largest being 

 2 or 3 mm. in diameter) that it diminished rapidly in size and at a foot in depth 

 was frequently indistinguishable from its branches. Most of these laterals 

 were found to spread horizontally in the surface 0.3 to 0.4 foot of soil to dis- 

 tances of 0.5 to 1.5 feet before ending, or, as is more usual, turning downward. 

 Below 6 inches no large branches were given off, nor is the tap-root or its 

 deeply descending branches furnished so abundantly with finely rebranched 

 rootlets below a depth of 1.2 feet, as in the surface soil. Most of the many 

 descending branches ended in the third or fourth foot and none was traced to 

 a greater depth than 5 feet. In general, this root habit agrees very closely 

 with that of the specimens excavated at Colorado Springs (Weaver, 1919:49). 

 The chief difference is the greater root penetration (5 to 6.5 feet) and the 

 wider surface spread of the laterals (0.3 to 2 feet) of the more robust plants 

 at the latter station. 



Petalostemon purpureus. — This species of prairie clover, like P. candidus, 

 has a very wide distribution. Both form societies, being especially con- 

 spicuous in late summer. They are best developed in true prairie and are also 

 abundant in mixed prairie, but seldom occur in the short-grass plains. 



A very robust specimen, with a tap-root 2 cm. in diameter, was examined in 

 the sandhills near Yuma. It gave off 10 branches, ranging from 4 to 7 mm. 

 in thickness in the first 8 inches of soil. These spread widely in the shallow 

 soil, as is characteristic of the species, before turning downward. Like the 

 tap-root, the laterals taper very rapidly, and througliout their horizontal 

 course give off an abundance of both large and small sublaterals. Upon turn- 

 ing downward, like the tap, they follow a more or less vertical course, but 

 curve backward and forward through distances of from several inches to more 

 than a foot. They taper so rapidly that below 1.5 feet none of the roots are 



