Schlindwein C., Wittmann D., Stamen movements in flowers of Opuntia (Cactaceae) favour oligolectic pollinators, Plant Systematics and Evolution, 1997, 204(3-4):179-193, doi: 10.1007/BF00989204
Studies of the flowers of two southern Brazilian prickly pears, published as O. brunneogemmia and O. viridirubra. They describe stamen movement after touch, the layered distribution of pollen, bee behaviour and the results of pollination with self and cross pollen.