Pollination biology of Canna indica (Cannaceae) with particular reference to the functional morphology of the style

Abstract

The anatomy of the bizarre style of Canna indica is analyzed functionally and comparatively within Zingiberales, particularly in relation to the presence of two stigma-like areas, one apical and the other lateral and subapical. We asked whether these areas have separate receptive or adhesive functions and whether they are derived from a single stigma that previously had both functions. We expected that the mechanism of pollen transference would be highly effective at the flower level, i.e., that pollen limitation would affect fruit set rather than seed set. Both areas produce a sticky carbohydrate- and mucilage-positive exudate but only the apical one leads to pollen tube growth into the stylar canal. The subapical and lateral area is regarded here as being homologous to the viscidium of Lowiaceae and Marantaceae. High pollen limitation through fruit set is attributable to the low visitation rate of the single long-billed hummingbird pollinator (Heliomaster furcifer) and to pollen loss caused by nectar robbers, while low limitation through seed set suggests that the efficiency of a few visits of pollen-carrying hummingbirds is high. Recordings of the pollination process indicate that the viscidium is touched before the pollen presenter, when hummingbirds are flying out of the flowers.