Functional Dioecism in the Malpighiaceae: The Breeding System of Spachea Membranacea Cuatr

The breeding system of Spachea membranacea (Malpighiaceae), an understory tropical rain forest tree, was studied on Barro Colorado Island in central Panama. Two types of trees were found, those with hermaphroditic flowers and those with female flowers. Crossing experiments indicated that morphological hermaphrodites normally function as males but may be induced to set fruit if emasculated prior to cross‐pollination. Thus, although morphologically gynodioecious, S. membranacea is functionally dioecious. This is the first experimental evidence for functional dioecism in the Malpighiaceae. The proportions of hermaphrodites (functional males) in the three populations examined were 44.7% (N = 47), 60.0% (N = 10), and 87.5% (N = 17). The distribution of hermaphrodites and females in size classes was quite similar, suggesting that sexual expression is not influenced by the age of the individual. Flowers are visited and presumably pollinated primarily by bees of the genera Paratetrapedia (Anthophoridae) and Trigona (Apidae).