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

摘要: This article, based on serial sections from 19 species of Ludwigia (supplemented where necessary with preparations other Onagraceae), begins an effort to outline the evolution flower, fruit, and seed characters within genus link what is known floral elsewhere in family. New observations include following: all anthers have a prominent endothecium, developing certain advanced are markedly H-shaped cross section; pollen two matures isolated packets; ovules L. leptocarpa, though commonly 1-seriate, can be distally pluriseriate; only rarely does placenta median groove suggesting paired carpel margins.' The deeply intrusive placentas seen section Myrtocarpus, but lacking some sections, probably ancestral, old idea that diplostemony 4+-mery ancestral holds up well when reexamined critically. Few families been as intensively studied by evolutionary botanists Onagraceae. Relationships among many infraspecific variants closely connected groups firmly established through cytological work, breeding experiments, field reproductive events. As one proceeds more widely separated taxa, however, biosystematic methods become inapplicable; consequently, links genera Onagraceae not yet understood. Structural comparison remains bestperhaps only-way improve our knowledge these links. First, structural differences taxa must identified, then direction change inferred critically weighing alternatives. ideal several respects for comparative studies structure. For thing, family manageable size: Raven currently recognizes 17 estimates number 600-700. "Spirit collections" available anatomical work because research efforts his collaborators. Another advantage working diverse enough challenging, undoubtedly common origin. Among show natural peculiar viscin threads onagraceous (Skvarla et al., 1977) distinctive 4-nucleate embryo sac (Seshavataram, 1970; Bhatnagar & Johri, 1972:91; Palser, 1975:641). Still another nearest extra-familial affinities myrtalean Combretaceae, Crypteroniaceae, Lythraceae, Melastomataceae, Myrtaceae, Punicaceae, Sonneratiaceae. Similarities structure this alliance were recognized pre-Darwinian taxonomists now indicators shared ancestry, strong con1 I thank P. T. Ramamoorthy criticizing typescript. National Science Foundation contributed indirectly, via series grants Raven, supporting collectors. Photographs Smithsonian photographer V. Krantz museum specialist S. Yankowski, respectively. 2 Department Botany, Institution, Washington, D.C. 20560. ANN. Missoum BOT. GARD. 64: 644-655. 1977. content downloaded 157.55.39.138 Mon, 27 Jun 2016 07:10:40 UTC All use subject http://about.jstor.org/terms 1977] EYDE-EVOLUTION IN LUDWIGIA 645 firmatory evidence such sources embryology (Subramanyam, 1951) vegetative anatomy (Carlquist, 1975; van Vliet, Vliet Baas, 1975). Ideas "ancestral versus derived" tested looking into related satisfactory distribution putative an-