SUMMARY. 145 



This is in sharp contrast to the usual root habit of plants of the more 

 xerophilous grassland associations, which, apparently because of drier 

 subsoil and a consequent greater dependence for moisture on the sur- 

 face soil layers, show, in addition to great depth of penetration, a highly 

 developed root system for absorption in the surface soil. True-prairie 

 species in lowland areas usually show a greater shoot and less root 

 development than when growing in upland soil. This seems to be a 

 response to the uniformly higher water-content of air and soil in the 

 lowland. 



A comparison of the factors affecting growth in the true prairie at 

 Lincoln, Nebraska, with the mixed prairie at Colorado Springs, Col- 

 orado, reveals the fact that conditions in the former habitat are more 

 favorable in every respect. At Lincoln the mean annual precipita- 

 tion is 28.6 inches, at Colorado Springs only 14.6. Run-off is greater, 

 and hence the rainfall is less efficient at the latter station, because of the 

 more compact soil. At both stations the soil is fertile and very deep, 

 and water-content of soil and air is the limiting factor of plant-growth. 

 Soil-moisture studies at Lincoln from 1916 to 1919, together with those 

 for other periods (Alway et ah, 1919) show that there is a constant supply 

 of water in the subsoil even during the driest years, and normally water 

 is available at all depths. Water-content determinations at Colorado 

 Springs during 1918 and 1919 show that the available soil-moisture at 

 all depths is much less and that frequently the first foot of soil has no 

 water available for growth, while data from other mixed-prairie areas 

 (Alway et al., at McCook, Nebraska) indicate that during dry phases 

 of the climatic cycle the available water supply is exhausted to a depth 

 of 3 or 4 feet. 



The evaporating power of the air is much higher in the mixed prairie 

 than at Lincoln, frequently being twice as great and sometimes reaching 

 45 to 60 c. c. per day. In true prairie the average daily evaporation 

 throughout the growing-season scarcely exceeds 22 c. c. and a maximum 

 of 35 c. c. is seldom reached. 



The mean day and night temperatures throughout the growing- 

 season are, with few exceptions, more favorable for plant development 

 in the true prairie, while the fluctuations between day and night ex- 

 tremes are much less at Lincoln. In the mixed prairie these range 

 from 35° to 40° F., the temperature varying from 90° to 95° F. in the 

 shade in the afternoon to 50° to 60° F. in the morning. Even greater 

 temperature extremes occur in the surface soil in the mixed prairie, 

 ranging from 55° to 60° F. in early morning to 120° or 125° F. in the 

 afternoon. These habitat differences affect not only the abundance 

 and height-growth of the species, but also profoundly modify the root 

 development. 



Among the 36 opecies of plants examined in the hard lands of mixed 

 prairie at Colorado Springs, in north central Kansas, and western 



