Pistil Structure of Banksia menziesii R.Br. (Proteaceae) in Relation to Fertility

Abstract

The morphology and histochemistry of the pistil of Banksia menziesii were studied to determine whether the observed low fertility of the species has a structural basis. The distal portion of the style, the pollen presenter, was distinct in both external and internal morphology from the remainder of the style. It was an elongated structure with eight longitudinal ridges, a swollen base, and with pollen-receptive stigma cells enclosed within a groove located at its tip. The stigma was wet papillate, and was covered with a lipidic secretion overlying polysaccharide-rich mucilage. Below the groove, the solid transmitting tract comprised a few thick-walled cells surrounded by numerous transfer cells, and vascular bundles associated with sclerenchyma. The style immediately below the pollen presenter was constricted, and the transfer cells ended in this region. The upper stylar cortex consisted largely of sclerenchyma, and the transmitting tract narrowed towards the base, comprising only 10.9 ± 0.3 cells at the junction with the ovary. The ovary contained two ovules, both of which were functional at anthesis, although the upper ovule developed more quickly than the lower. The inflorescence consisted of over 700 flowers, of which 2.5 ± 0.2% had short styles and pollen presenters with exposed stigma papillae. Apart from this, there was no variation in pistil structure which would explain the observed low fertility.