Floral rewards: alternatives to pollen and nectar

In angiosperms selection has led to the utilization of numerous substances other than pollen and nectar that help to insure repeated visitation by pollinating animals. Here, we group the various substances into nonnutritive and nutritive rewards and discuss within each group the specific kinds that occur. In our discussion of nutritive rewards, we emphasize floral oils, lipids produced by one of two types of specialized secretory organs called elaiophores and which serve as nutritive rewards for certain New World anthophorine bees. Although discovered only within the last 15 years, the syndrome of oil production now appears to be one of the most widespread kinds of floral rewards. We report here for the first time the occurrence of oil production in the Solanaceae (Nierembergia). It is apparent that oil production has evolved independently many times, but plants which produce oils that are collected by female anthophorine bees show similarities in the chemistry of the oils and the types of structures that produce them. It is not clear whether other groups of plants reported to produce oils but which are not pollinated by anthophorine bees possess an analogous system or not.