Floral Development in the Nymphaeales

Abstract

Numerous phylogenetic analyses and recent paleobotanical findings continue to support the inclusion of Nymphaeales as basal or near‐basal extant angiosperms. The distinctive differences in floral morphology and habit between non‐Nymphaealian taxa of the ANITA grade compared with water lilies, nevertheless, remain perplexing. In this investigation, scanning electron microscope observations on Brasenia (Cabombaceae), Nuphar, Nymphaea, Ondinea, Victoria, and Euryale (Nymphaeaceae) are added to the observations of earlier investigations to enhance our understanding of early floral evolution and shifts in floral ontogeny and to contribute new data for improved cladistic analyses. The enlargement of flowers and amplification of floral parts distinctive in Nymphaeaceae is based on continuing surface area enlargement of the floral apex, allowing numerous organs to be produced acropetally in distinctive parastichies, compared with the ring meristem found in some primitive eudicots (Nelumbonaceae, Papaveraceae) that increase floral organ number but in dense pulses or clusters without clear order. The smaller flowers of the Cabombaceae and their lower number of floral organs are now regarded as more typical of the ancestral angiosperm flower. One may conclude that changes in the size of the floral apex and its geometric conformation alter flower size, number of parts, and phyllotaxy. The ebracteate condition and herbaceous, aquatic habit of Nymphaeales when compared with Archaefructus, a recently discovered herbaceous aquatic angiosperm from the Upper Jurassic/Lower Cretaceous that is proposed as a new basal angiosperm family, continue to question the relationship of early angiosperms to the aquatic habitat. Is the ebracteate state significant, and how did the shift to aquatic habitats impact floral form of early angiosperms?