Heterostyly in the Connaraceae with Special Reference to Byrsocarpus coccineus

Abstract

1. Since 1887, tristyly has been claimed repeatedly as typical of the Connaraceae, with distyly and dioecism as derived conditions produced by sterilization of androecial or carpellary parts. 2. In the African genus Byrsocarpus, B. puniceus (long-styled) and B. coccineus (short-styled) are now regarded as forms of the same species (B. coccineus Schum. et Thonn.). Despite distyly, the two whorls of stamens in each form are roughly equal in length and all are polleniferous. No differences in size of pollen grains were detectable either between flower forms or between whorls in the same flower. 3. Both long-styled and short-styled plants have twenty-eight somatic chromosomes. 4. Both forms are partially self-incompatible and outbreed by crossing with each other. Even self-pollination requires some disturbance of the flower, and butterflies, wasps, and flies are potential pollinators. Indirect evidence of outcrossing is given by roughly equal numbers of individuals of each form in natural populations as well as by interplant variability. Vegetative reproduction is also important in nature. 5. With this evidence, in B. coccineus, of regular distyly not accompanied by sterilization of a whorl of stamens, reports of tristyly elsewhere in the family are in need of confirmation, as are those of incipient dioecism. Studies on living material in the tropics will provide the most reliable data.