Floral anatomy in Dypsis (Arecaceae-Areceae): A case of complex synorganization and stamen reduction

Abstract

Male (staminate) flowers of Dypsis possess either six or three stamens and/or staminodes, in contrast to most other palms, in which the basic stamen number is six (although polyandry is also common in palms). Significant variation among the tristaminate forms, both in stamen morphology and stamen position with respect to the perianth parts, indicates that stamen reduction from six to three has occurred more than once, and possibly several times within Dypsis. A few species include teratological forms with zygomorphic flowers; for example, Dypsis lantzeana normally possesses three antesepalous stamens, but in some specimens only the median outer (abaxial) stamen is expressed, perhaps indicating incipient zygomorphy correlated with complex synorganization. Inclusion of earlier historic genera such as Neophloga and Chrysalidocarpus within a broadly circumscribed Dypsis appears to be justified, although the informal taxonomic groupings within Dypsis require review, in particular the taxonomic significance of the different types of anther morphology. The discovery here that adnation of staminodes to the pistillode (complex syn-organization) occurs in species other than D. mirabilis opens further questions about the taxonomic utility of this character in Dypsis, in which stamen/staminode development and adnation are apparently unusually labile. Such androecial-gynoecial adnation is otherwise rare in palms, as also in other monocots, in which probably the best-known example occurs in orchids. Septal nectaries are present in some, but not all, staminate flowers in species of Dypsis. Dypsis bejofo is exceptional in that in staminate flowers the pistillode is distally bulbous and bears three prominent modified supralocular septal nectaries. Female (pistillate) flowers in Dypsis are syncarpous, normally pseudomonomerous (as in many other Arecoideae), and possess septal nectaries that effectively delimit the carpel margins and indicate insect pollination. There is a central solid transmitting tissue that extends from the placenta to three stylar canals. The stylar canals of the two sterile carpels are apparently functional, in addition to that of the fertile carpel. © 2003 The Linnean Society of London.