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

Abstract

Based on serial sections from 24 spp. of Ludwigia (supplemented where necessary with preparations from other Onagraceae), the evolution of flower, fruit and seed characters within the genus was outlined. The outline was linked with floral evolution elsewhere in the family. All Ludwigia anthers have a prominent endothecium, and developing anthers of certain advanced species are markedly H-shaped in cross section. Pollen of 2 spp. matures in isolated packets. Ovules of L. leptocarpa, though commonly I-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.