Structure and Histochemistry of the Stigma and Style of Some Australian Species of Acacia

Abstract

The structure and histochemistry of the stigma and style of Acacia conferta, A , dealbata, A . iteaphylla, A . mearnsii, A. retinodes and A. subulata were investigated in relation to the pathway of pollination. The stigma is non-papillate and forms a cup-shaped depression at the tip of the style that glistens with exudate at anthesis. This exudate is heterogeneous in microscopic appearance and stained positively for proteins, carbohydrates and lipids. No cuticle is detectable over the stigma surface and living stigma cells showed a high permeability to staining by both the periodic acid-Schiff's reaction and ruthenium red. Esterase activity, detected cytochemically in fresh stigmas, is present in the surface exudate and in the walls of the transmitting cells in the upper style. The style has a central transmitting tissue, consisting of longitudinally elongated rows of cells loosely packed in a mucilaginous extracellular matrix. The cytoplasm of the transmitting cells stained intensely for proteins.