1. Bartholomew, B.: Drought response in the gas exchange of Dudleya farinosa (Crassulaceae) grown under natural conditions. Photosyn. 7, 114–120 (1973)
2. Bement, R.E.: Plains pricklypear: Relation to grazing intensity and blue grama yield on central Great Plains. J. Range Manage. 21, 83–86 (1968)
3. Chow, P.C., Burnside, O.C., Lavy, T.L.: Physiological studies with prickly-pear. Weeds 14, 58–62 (1966)
4. Dickinson, C.E., Dodd, J.L.: Phenological pattern in the shortgrass prairie. Am. Mid. Nat. 96, 367–378 (1976)
5. Dodd, J.L., Lauenroth, W.K.: Responses of Opuntia polyacantha to water and nitrogen perturbations in the shortgrass prairie. In: Prairie: A multiple view (M.K. Wali, ed.), pp. 229–240. Grand Forks, N.D.: North Dakota Press 1975