Reproductive morphology of Sargentodoxa cuneata (Lardizabalaceae) and its systematic implications

The reproductive morphology of Sargentodoxa cuneata (Oliv) Rehd. et Wils. is investigated through field, herbarium, and laboratory observations. Sargentodoxa may be either dioecious or monoecious. The functionally unisexual flowers are morphologically bisexual, at least developmentally. The anther is tetrasporangiate, and its wall, of which the development follows the basic type, is composed of an epidermis, endothecium, two middle layers, and a tapetum. The tapetum is of the glandular type. Microspore cytokinesis is simultaneous, and the microspore tetrads are tetrahedral. Pollen grains are two-celled when shed. The mature ovule is crassinucellate and bitegmic, and the micropyle is formed only by the inner integument. Megasporocytes undergo meiosis resulting in the formation of four megaspores in a linear tetrad. The functional megaspore develops into an eight-nucleate embryo sac after three rounds of mitosis. The mature embryo sac consists of an egg apparatus (an egg and two synergids), a central cell, and three antipodal cells. The pattern of the embryo sac development follows a monosporic Polygonum type. Comparisons with allied groups show that Sargentodoxa shares more synapomorphies with the Lardizabalaceae than other Ranunculales. Characteristics of its reproductive morphology are consistent with the placement of Sargentodoxa as the sister group of the remaining Lardizabalaceae. It does not possess a sufficient number of apomorphic characters to justify its separation into a separate family or subfamily. It is best retained as a member of the Lardizabalaceae.