soil moisture is low in the spring although ample later in the sea- 

 son, Bromus tectorum cannot germinate and grow well, with result- 

 ing reduction in abundance and height; but Agropyron smithii and 

 Bouteloua gracilis are able to use the moisture that comes in later 

 rains and therefore grow much better in the absence of severe 

 competition with Bromus tectorum. It has been found that the 

 germination of B. tectorum seeds is retarded by certain tempera- 

 ture conditions,' ^2 which may be a limiting factor in certain years 

 for this bromegrass in this community. 



GENERAL REFERENCES 



AUee, W. C, et ai, "Principles of Animal Ecology," W. B. Saunders Co., Phila- 

 delphia, Pa., 1949. 



Anderson, E., "Introgressive Hybridization," John Wiley & Sons, Inc., New 

 York, N.Y., 1949. 



Conard, H. S., "The Background of Plant Ecology" (trans, from the German 

 of Kerner, A., 1863, "The Plant Life in the Danube Basin"), Iowa State Col. 

 Press, Ames, Iowa, 1951. 



Dahl, E., "Rondane, Mountain Vegetation in South Norway and Its Relation 

 to the Environment," Aschehoug & Co., Oslo, 1956. 



Tansley, A. G., "The British Islands and their Vegetation," 2 Vols., Cambridge 

 Univ. Press, Cambridge, 1949. 



I 



74 • Species axidl Popvmlatioxks 



