The connective base and filament of Acicarpha tribuloides (Calyceraceae)

Abstract

The structural and autofluorescent properties of the staminal connective base of Acicarpha tribuloides (Juss.) clearly distinguish it from the filament. The abaxial and lateral connective base epidermal walls have secondary thickenings and are autofluorescent. The filament walls lack these characteristics. Also, the wrinkled or bulbous cuticular surface of the filament epidermis is distinct from that of the connective base which is smooth and relatively flat. In regard to these characteristics, A. tribuloides strongly resembles Cirsium horridulum (Michx.), a species in the Asteraceae. A. tribuloides differs from C. horridulum in that the cytoplasm in both the connective base and the free filament cells becomes severely disorganized and some of the walls collapse during anthesis in the former species. Thus, while the staminal tissues of these two species have some similar structural and biochemical features, they have significantly different functional roles. The presence of a connective base in the Calyceraceae significantly broadens the known range of this feature, and can be used as evidence to support the hypothesis that this feature is plesiomorphic within the Asteraceae.