Springer Science and Business Media LLC

Opuntia brunneogemmia andO. viridirubra occur sympatrically in the Serra do Sudeste, Rio Grande do Sul, Brazil. Their flowers have 450–600 thigmonastic stamens and provide large amounts of pollen and nectar for bees. Bees of 41 species were registered at the flowers ofO. brunneogemmia and 30 at the flowers ofO. viridirubra. Females of three oligolectic species are the only effective pollinators:Ptilothrix fructifera (Anthophoridae),Lithurgus rufiventris (Megachilidae), andCephalocolletes rugata (Colletidae). During their visits inOpuntia-flowers, bees touch the filaments and stimulate the movement of the stamens to the centre of the flower. At the end of this movement, the anthers are densely packed around the style. As a consequence the pollen is presented in an easily accessible upper layer of anthers and various, nearly inaccessible lower layers. The lower layers contain about 80% of the pollen reward. Only females of the three oligolectic pollinators exploit the pollen from the lower layers and reach the nectar furrow. Therefore, through their stamen movements,Opuntia flowers hide most of their pollen from flower visitors but favour effectively pollinating, oligolectic bees.

Pollination of the alpine herbDianthus glacialis was studied in a population in the Swiss Alps in 1991 and 1992. Only one insect species,Zygaena exulans (Lepidoptera), was observed to visit the flowers ofD. glacialis. Pollen loads onZ. exulans indicate that it is an effective pollinator ofD. glacialis. In 1991,Z. exulans frequently visited flowers ofD. glacialis. However, in 1992 almost no visits could be observed. Despite the occurrence of pollinators and the conspicuous flowers ofD. glacialis, there are strong indications that this plant is mainly selfing: (1) anthesis of individual flowers is short (c. one and a half days), (2) protandry, although pronounced in otherDianthus spp., is absent, (3) nectar production is low, (4) odour production is weak, (5) seed production by spontaneous selfing is high, (6) seed set in emasculated flowers is extremely low and (7) the seed/ovule ratio is distinctly higher inD. glacialis than in outcrossing plants.—Selfing inD. glacialis could have evolved under a shortage of pollinators during the glaciation periods. Present levels of cross-pollination byZ. exulans have apparently not been sufficient to reverse the pollination mechanism ofD. glacialis from selfing to outcrossing.