Koch K. E., Kennedy R. A., Effects of seasonal changes in the Midwest on Crassulacean Acid Metabolism (CAM) in Opuntia humifusa Raf., Oecologia, 1980, 45(3):390-395, doi: 10.1007/BF00540212
The study of the annual cycle of plants from Iowa combines measurements of gas exchange, acidity and hydration. It describes seasonal variation in nocturnal CO₂ uptake in material identified in 1980 as Opuntia humifusa.