The floral morphology of seven Oxypetalum species and, in particular, the spatial relationship between the five stigmatic chambers and two separate ovaries of their flowers with respect to transmission of the pollen tube are studied. In all species, except O. banksii subsp. banksii, floral morphology is similar to that in other Asclepiadeae, and the flowers pollinated with one pollinium develop only one follicle, which means compitum absence. In O. banksii subsp. banksii flowers, the secretory interstaminal tissue lines the inner walls of the stigmatic chambers as in the other species studied, but it also reaches the upper part of the inner surface of the filament tube, where it surrounds the styles, an unprecedented feature for Asclepiadaceae. This tissue secretes nectar and mucilage; the latter acts as transmitting medium for the growth of pollen tubes from pollinia inserted and hydrated in stigmatic chambers ("hyperstigmas"). Mucilage also functions as an extragynoecial compitum: in flowers pollinated with one pollinium both carpels develop into a follicle.2333-4199206Baker, H.G., Baker, I., Floral nectar sugar constitutents in relation to pollinator type (1983) Handbook of Experimental Pollination Biology, pp. 117-141. , Jones C. E., Little R. J. (eds.). Van Nostrand Reinhold Company Inc., New YorkBookman, S.S., The floral morphology of Asclepias speciosa (Asclepiadaceae) in relation to pollination and a clarification in terminology for the genus (1981) Amer. J. Bot., 68, pp. 675-679Carr, S.G.M., Carr, D.J., The functional significance of syncarpy (1961) Phytomorphology, 11, pp. 249-256Christ, P., Schnepf, E., The nectaries of Cynanchum vincetoxicum (Asclepiadaceae) (1985) Israel J. Bot., 34, pp. 79-90Cronquist, A., (1988) The Evolution and Classification of Flowering Plants, , Allen Press, LawrenceEndress, M.E., Bruyns, P.V., A revised classification of the Apocynaceae s.l. (2000) Bot. Rev., 66, pp. 1-56Endress, P.K., Noncarpellary pollination and "hyperstigma" in an angiosperm (Tambourissa religiosa, Monimiaceae) (1979) Experientia, 35, p. 45Endress, P.K., Ontogeny, function and evolution of extreme floral construction in Monimiaceae (1980) Plant Syst. Evol., 134, pp. 79-120Endress, P.K., Syncarpy and alternative modes of escaping disadvantages of apocarpy in primitive angiosperms (1982) Taxon, 31, pp. 48-52Endress, P.K., (1994) Diversity and Evolutionary Biology of Tropical Flowers, , Cambridge University Press, CambridgeFallen, M.E., Floral structure in the Apocynaceae: Morphological, functional, and evolutionary aspects (1986) Bot. Jahrb. Syst., 106, pp. 245-286Fontella-Pereira, J., Schwarz, E.D.A., Estudos em Asclepiadaceae, XIX. Uma nova espécie de Gonolobus Mich e novas combinações em Oxypetalum R (1984) Br. Bol. Mus. Bot. Munic. Curitiba, (61), pp. 1-8Fontella-Pereira, J., Valente, M.D.C., De Alencastro, F.M.M.R., Contribuição ao estudo das Asclepiadaceae brasileiras, V. Estudo taxonômico e anatômico de Oxypetalum banksii Roem. et Schult (1971) Rodriguésia, (38), pp. 261-281Fontella-Pereira, J., Valente, M.D.C., Schwarz, E.D.A., Contribuição ao estudo das Asclepiadaceae brasileiras, XXI. Asclepiadaceae do município de Ouro Preto, Estado de Minas Gerais - Uma sinopse (1984) Bol. Mus. Bot. Kuhlmann, (2), pp. 63-127Galil, J., Zeroni, M., Nectar system of Asclepias curassavica (1965) Bot. Gaz., 126, pp. 144-148Gerlach, G., (1969) Botanische Mikrotechnik, Eine Einführung, , Georg Thieme, StuttgartHolm, R.W., The American species of Sarcostemma R. Br. (Asclepiadaceae) (1950) Ann. Missouri Bot. Gard., 37, pp. 477-560Johansen, D.A., (1940) Plant Microtechnique, , McGraw-Hill, New YorkKevan, P.G., Eisikowitch, D., Rathwell, B., The role of nectar in the germination of pollen in Asclepias syriaca L. (1989) Bot. Gaz., 150, pp. 266-270Kunze, H., Structure and function in asclepiad pollination (1991) Plant Syst. Evol., 176, pp. 227-253Kunze, H., Evolution of the translator in Periplocaceae and Asclepiadaceae (1993) Plant Syst. Evol., 185, pp. 99-122Kunze, H., Morphology of the stamen in the Asclepiadaceae and its systematic relevance (1996) Bot. Jahrb. Syst., 118, pp. 547-579Kunze, H., Corona and nectar system in Asclepiadinae (Asclepiadaceae) (1997) Flora, 192, pp. 175-183Kunze, H., Liede, S., Observations on pollination in Sarcostemma (Asclepiadaceae) (1991) Plant Syst. Evol., 178, pp. 95-105Liede, S., Anther differentiation in the Asclepiadaceae - Asclepiadeae: Form and function (1996) The Anther: Form, Function and Phylogeny, pp. 221-235. , D'Arcy W. G., Keating R. C. (eds.). Cambridge University Press, CambridgeLiede, S., Albers, F., Tribal disposition of genera in the Asclepiadaceae (1994) Taxon, 43, pp. 201-231Martin, F.W., Staining and observing pollen tubes in style by means of fluorescence (1959) Stain Techn., 34, pp. 125-128Morse, D.H., The twinning of follicles by common milkweed (Asclepias syriaca) (1993) Amer. Midl. Nat., 130, pp. 56-61Occhioni, P., Notas sôbre o gênero Oxypetalum - II (As espécies do Estado do Rio de Janeiro) (1953) Dusenia, 4, pp. 251-271Sage, T.L., Williams, E.G., Structure, ultrastructure, and histochemistry of the pollen tube pathway in the milkweed Asclepias exaltata L. (1995) Sex. Plant Repr., 8, pp. 257-265Sage, T.L., Broyles, S.B., Wyatt, R., The relationship between the five stigmatic chambers and two ovaries of milkweeds flowers: A three-dimensional assessment (1990) Israel J. Bot., 39, pp. 187-196Sennblad, B., Bremer, B., The familial and subfamilial relationships of Apocynaceae and Asclepiadaceae evaluated with rbcL data (1996) Plant Syst. Evol., 202, pp. 153-175Sparrow, F.K., Pearson, N.L., Pollen compatibility in Asclepias syriaca (1948) J. Agr. Res., 77, pp. 187-199Swarupanandan, K., Mangaly, J.K., Sonny, T.K., Kishorekumar, K., Chand Basha, S., The subfamilial and tribal classification of the family Asclepiadaceae (1996) Bot. J. Linn. Soc., 120, pp. 327-369Vieira, M.F., (1998) Biologia Reprodutiva de Espécies de Oxypetalum (Asclepiadaceae) na Região de Viçosa, MG, Sudeste Brasileiro, , Ph. D. Thesis, Campinas, São Paulo, BrazilVieira, M.F., Shepherd, G.J., Pollinators of Oxypetalum (Asclepiadaceae) in southeastern Brazil (1999) Rev. Brasil. Biol., 59, pp. 693-704Vieira, M.F., Shepherd, G.J., Sistema reprodutivo de Oxypetalum appendiculatum Mart. e Oxypetalum banksii Roem. & Schult. subsp. banksii (Asclepiadaceae) (1999) Acta Bot. Bras., 13, pp. 237-241Vieira, M.F., Shepherd, G.J., Removal and insertion of pollinia in flowers of Oxypetalum (Asclepiadaceae) in southeastern Brazil (2002) Rev. Biol. Trop., p. 50. , in pressWalker, D.B., Postgenital carpel fusion in Catharanthus roseus (Apocynaceae). IV. Significance of the fusion (1978) Amer. J. Bot., 65, pp. 119-121Woodson, R.E., The North American species of Asclepias L. (1954) Ann. Missouri Bot. Gard., 41, pp. 1-211