Morphology, development and homologies of the perianth and floral nectaries in Croton and Astraea (Euphorbiaceae-Malpighiales)

Abstract

New observations are presented on the ontogeny, vasculature and morphology of both staminate and pistillate flowers of Croton and Astraea. These data support earlier hypotheses that the filamentous structures in pistillate flowers represent reduced and transformed petals. Staminate flowers of both genera possess five free nectaries, which are vascularised by divergences of the sepal traces in Croton and unvascularised in Astraea. In pistillate flowers, there are five separate non-vascularised nectaries in Astraea, but in Croton there is a single nectariferous disk that is vascularised by divergences of the sepal traces. The nectaries are initiated late in floral development, but their location indicates that they could represent the outer stamen whorl transformed into secretory staminodes. Other glandular structures occur in pistillate flowers of most Croton species, resulting in flowers with two secretory organ whorls. In these cases, the inner whorl is formed by modified staminodes. Our observations support the recent segregation of Astraea species from the larger genus Croton. Despite strong similarities between the two genera, there are clear structural differences, including the presence of colleters in Astraea (absent in Croton), moniliform trichomes on petals (rather than simple trichomes in Croton), non-vascularised nectaries (vascularised in Croton) and reduced, non-secretory filamentous structures (well developed and secretory in Croton).