The proteaceous pistil: Morphological and anatomical aspects of the pollen presenter and style of eight species across five genera

Abstract

The morphology and anatomy of pollen presenters, styles and pollen of species of Banksia, Dryandra, Hakea, Isopogon and Macadamia were studied. Serial sections of pistils and SEM images of pollen were quantified to determine whether the low fertility observed in the Proteaceae has a structural basis. Pollen access to the stigma was investigated. There were three types of stigmatic cavity. A groove in which the stigmatic papillae were enclosed was present in Dryandra, Banksia and Hakea. Macadamia had a groove with protruding papillae, and Isopogon had a tube which enclosed the papillae. Anatomical studies showed the pollen presenter to be structurally complex but overall to have similar internal anatomy across the species studied. The species could be grouped according to presence or absence of transfer tissue and presence or absence of sclerenchyma, but these groups were not mutually exclusive. In the pistil there were three structural filters to pollen tube passage. The first was at the stigma, where pollen grain access could be limited by the diameter or length of the stigmatic groove or the capacity of the pollination chamber in relation to pollen volume. The second and third related to a narrowing of the transmitting tissue tract within the pollen presenter and in the lower style which could influence pollen tube passage to the ovule.