THE SEXUALITY OF THE BARRO COLORADO ISLAND FLORA (PANAMA) Thomas B. Croat Missouri Botanical Garden 23^5 Tower Grove Avenue St. Louis, Mo. 63110 The sexual expression of species making up a flora have frequently been analyzed, but as yet no enumeration has been given for any entire neo-tropical flora. Probably the most thorough survey of the distribution of sexual expression was that made by Yampolsky and Yampolsky (1922), but that now much out of date survey was made on a world-wide phyllogenetic basis rather than on a geograph-ical basis. The recent completion of the Flora of Barro Colorado Island (Croat, 1978) affords the opportunity to report on the sexual behavior of a species-rich tropical forest in the isthmus of Panama. Barro Colorado Island, which lies in Gatun Lake midway between the Atlantic and Pacific Oceans, has a flora of 1,3^9 species of vascular plants. Although the area is small in size (ca 6 square miles), it is representative of tropical moist forest, which makes up approximately ^Ofo of the total area of Panama. Moreover, it occupies a central position between North and South America and apparently has acquired nearly equal numbers of species from the Central and South American floras (Croat and Busey, 1975). Of the 1,212 species of native phanerogams in the flora, 286 (2kfc) have unisexual flowers. Of these, 106 {9%) are dioecious, 132 (11^) are monoecious , (4 are monoecious or dioecious and are included in both categories), and 52 {^%) are polygamous . On BCI there are ^81 arborescent species (39.7^ of the native vascular flora) (See Table 1). Of this number, 3^■ {6,k%) are trees which may be larger than 30 ni and are possible emergents , while 177 (37^) are trees IO-30 m tall. The two groups inclusively are referred to here as medium to large trees. There are 265 climbing plants making up 20^ of the flora and 466 herbaceous plants (not counting herbaceous vines), accounting for 35*5:^ of the phanerogamic flora. 319
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