Reproductive Structures and Evolution in Ludwigia (Onagraceae). I. Androecium, Placentation, Merism

This article, based on serial sections from 19 species of Ludwigia (supplemented where necessary with preparations from other Onagraceae), begins an effort to outline the evolution of flower, fruit, and seed characters within the genus and to link the outline with what is known of floral evolution elsewhere in the family.New í include the following: all Ludwigia anthers have a prominent endothecium, and developing anthers of certain advanced species are markedly H-shaped in cross section; pollen of two species matures in isolated packets; ovules of L. leptocarpa, though commonly l-seriate, can be distally pluriseriate; only rarely does a Ludwigia placenta have a median groove suggesting paired carpel margins.The deeply intrusive placentas seen in section Myrtocarpus, but lacking in some of the other sections, are probably ancestral, and the old idea that diplostemony and 4+-mery are ancestral holds up well when reexamined critically.Few families have been as intensively studied by evolutionary botanists as the Onagraceae.Relationships among many infraspecific variants and among closely connected species groups have been firmly established through cytological work, breeding experiments, and field observations of reproductive events.As one proceeds to more widely separated taxa, however, biosystematic methods become inapplicable; consequently, evolutionary links among the genera of Onagraceae are not yet well understood.Structural comparison remains the bestperhaps the only-way to improve our knowledge of these links.First, structural differences among the taxa must be identified, then the direction of evolutionary change can be inferred by critically weighing the alternatives.The Onagraceae are ideal in several respects for comparative studies of floral structure.For one thing, the family is of manageable size: Raven currently recognizes 17 genera and estimates the number of species to be 600-700."Spirit collections" of many of these species are available for anatomical work because of the research efforts of Raven and his collaborators.Another advantage in working with Onagraceae is that the taxa are diverse enough to be challenging, yet undoubtedly of common evolutionary origin.Among the characters that show the Onagraceae to be a natural family are the peculiar viscin threads on onagraceous pollen (Skvarla et al., 1977) and the distinctive 4-nucleate embryo sac (Seshavataram, 1970; Bhatnagar & Johri, 1972:91; Palser, 1975:641).Still another advantage is that the nearest extra-familial affinities of the Onagraceae a known to be among the myrtalean families Combretaceae, Crypteroniaceae, Lythraceae, Melastomataceae, Myrtaceae, Punicaceae, and Sonneratiaceae.Similarities in floral structure within this alliance were recognized by pre-Darwinian taxonomists and are now seen as indicators of shared ancestry, with strong con-=c 11 thank P. Raven and T. P.