Structure and histochemistry of the stigma and style of Metrosideros excelsa

Abstract

The anatomy and histochemistry of pollinated and unpollinated pistils of New Zealand pohutukawa (Metrosideros excelsa) were examined. The wet stigma consisted of unicellular papillae. The style comprised a cutinised epidermal layer, cortical tissue, and a central triangular transmitting tissue with loosely arranged cells. The cells of the transmitting tract were surrounded by large intercellular spaces filled with mucilage. These spaces increased progressively in size towards the centre of the transmitting tract. The stigmatic exudate of unpollinated pistils stained intensely for carbohydrates, lipids, and proteins. In the style, up to 50% of cortical cells contained starch grains and the intercellular mucilage of the transmitting tissue stained weakly only for polyanions and pectins. Following pollination, the stigmatic exudate showed a notable decrease in lipids and proteins, and starch grains were depleted in the stylar cortex. However, with respect to the large number of pollen tubes travelling through the transmitting tissue, stylar resources are poor and this may provide an explanation for their slow growth rate.