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The unique design of pollen tetrads in Dionaea and Drosera

Abstract The heteropolar pollen grains of Dionaea and Drosera (Droseraceae) are united into permanent tetrads. Dionaea and Drosera tetrads have unique, complex internal set-ups. In both genera, apertures (pores) are situated equatorially (stephanoporate); neither Dionaea nor Drosera can be designated as examples for proximal polar situated apertures. In turgescent state, the tetrads show protrusions along the borders of adjoining pollen grains, which are enclosed by a thick intine and emerge from the apertures. One of these protrusions finally forms the pollen tube during germination. Both genera produce Ubisch bodies. In Drosera, the distal pollen wall above the apertures of each monad invaginates towards the centre of the pollen grain, forming an incomplete septum with a central opening. This septum divides the pollen grain into a larger distal and a smaller proximal chamber containing cytoplasm. In the literature, the septal opening was erroneously addressed as aperture (‘pore’). The proximal chambers collapse in the dry state (or after acetolysis) due to the harmomegathic effect. The septum becomes folded and partially in contact with the proximal wall, thus forming some sort of channels. The thick equatorial pollen walls of Dionaea muscipula show a row of pores, i.e. the apertures, from where the protrusions extend. In contrast to Drosera, there is no inner septum. Each protrusion in Dionaea comprises a massive operculum, which acts as a plug in dry pollen grains.