THE FLORAL BIOLOGY OF CASSIA DIDYMOBOTRYA AND C. AURICULATA (CAESALPINIACEAE)

The flowers of Cassia didymobotrya and C. auriculata have three types of fertile anthers that differ in orientation, size, shape, and pollen production. Flowers with right‐handed or left‐handed style deflections, i.e., enantiostylous flowers, occur in the same raceme. In Israel, both species are pollinated by pollen collecting buzzing females of Xylocopa pubescens. Vibrations of the carpenter bee are necessary for release of pollen from poricidal anthers and, in C. didymobotrya, also for absorption of pollen through the stigmatic opening into a cavity of the style tip. The largest anthers supply pollen for pollination. Pollen for consumption by the bee, on the other hand, appears to be supplied by all fertile anthers. This is in contrast to the accepted view of a complete separation of functions in the heteromorphic anthers in Cassia. The pollen/ovule ratio in C. didymobotrya was 55,200, and that in C. auriculata 32,000. Only 0.01% of the pollen produced by a flower was found in the stigmatic pollen load, and this fraction allows for high seed sets. From a comparison of the pollen production and utilization in the Cassia species and in certain distylous plants, it is concluded that heteranthery in Cassia does not confer an advantage in pollen economy. The high pollen/ovule ratios are explained by lack of nectar as well as by the large area on which pollen is deposited on the bee's body relative to the small size of the stigma. Enantiostyly in Cassia is considered as part of a pollination syndrome whose characteristics are outlined. The function of enantiostyly as an outbreeding strategy is discussed. In the two Cassia species, opportunities for self‐ and geitonogamous pollinations and self‐compatibility minimize an effect of enantiostyly in promotion of outcrossing. Instead, it is proposed that style deflections may clear access of the pollinator to the anthers and protect female parts from injury by a vibrating heavy‐bodied visitor.