34 



ROOT DEVELOPMENT IN THE GRASSLAND FORMATION. 



at a depth of 8 inches, the daily fluctuation was usually about 10° F. 

 within the range of 70° to 85° F. The temperature of the surface soil 

 shows an extreme variation. Not infrequently it ranges from 55° or 

 60° F. in early morning to 120° or 125° F. in the afternoon. Relatively 

 greater insolation resulting from a sparser cover of vegetation to ab- 

 sorb radiant energy and an average lower specific heat of the drier, 

 dark-colored soil, both combine to increase the day soil temperatures, 

 especially of the more superficial layers, over those of true prairie. 

 Such variations must have considerable influence upon the germination 

 and ecesis of such species as Koderia cristata, which may start growth 

 in June upon the ripening of seed. At Lincoln, the soil at a depth of 



Fig. 10. 

 A meter quadrat in the mixed 

 prairie community at Colo- 

 rado Springs, Colorado. A, 

 Andropogon scoparius; Af, 

 Artemisia frigida; Ar, Aris- 

 tida purpurea; \$\, Boute- 

 loua gracilis; Be, Boutcloua 

 curtipendula; Bo, Bozbera 

 papposa; C, Car ex penn- 

 sylvanica; Ch, Chrysopsis 

 villosa; K, Kceleria cristata; 

 L, Lithospermum linearifol- 

 ium; //// ', Muhlenbergia 

 gracillima; P, Psoralea ten- 

 uiflora; Ps, Pentstemon 

 angustifolius; R, Ratibida 

 columnar is. 



6 inches showed an average daily temperature range of 5° to 7° F. at 

 the low and high prairie stations respectively; the soil temperatures 

 varied from an average minimum of about 65° F. at the low-prairie 

 station in early summer to a maximum of about 85° F. at the high- 

 prairie station in August. 



Comparatively few investigations have been made on the evaluation 

 of soil temperature as an ecological factor in root growth. The work 

 of Cannon (1918) on certain desert species is indicative of the import- 

 ance of this factor not only in the development of the root system, but 

 also in the general distribution of vegetation. Thus it has been shown 

 that the general distribution of the cacti as a family is closely related 

 to the response of the roots to the temperature of the soil. Even in 

 the latitude of southern Arizona the conditions of soil temperature for 

 most favorable water absorption do not prevail in winter, and the 



