Flowers of Halgania (Ehretiaceae, Boraginales) are set up for being buzzed and the role of intertwining anther trichomes

Flowers that release their powdery pollen by being vibrated through buzzing are morphologically diverse and have evolved many times independently. Most prominently, buzz-pollination is known from Solanum (Solanaceae) defining the “solanoid” flower type by Knut Fægri. The flowers of buzz-pollinated, Australian Halgania resemble the actinomorphic Solanum floral architecture: The plants have a yellow anther cone with an apical pore, protruding from a widely funnel-shaped to rotate, blue to violet corolla. However, Fægri rejected the solanoid flower type for some of the Halgania species due to a contracted inflorescence, a funnel-shaped corolla or an ostensible lack of a female stage. Using light and scanning electron microscopy, we investigated traits referring to the floral architecture of Halgania and compared the flowers of different species. In all investigated species the anthers of the cone are connected via long, intertwined trichomes (denoted “capillinection”) both between the individual anthers and in the central stylar channel. The anther opening is functionally reduced to an apical pore by 1) capillinection and spatial compression of the thecal walls, leaving only small apical slits and 2) by anther appendages that unify the ten slits to a single pore. Halgania flowers are morphologically similar to each other and are buzz-pollinated. We conclude that they are all structurally and functionally matching the flowers of tomato, Solanum lycopersicum, and thus the concept of solanoid flowers as defined by Knut Fægri.