The subglandular tissue of Hibiscus rosa-sinensis nectaries

Abstract

Summary The nectary of Hibiscus rosa–sinensis is subtended by subglandular parenchymatous tissue extending from the bases of the nectary hairs to the depth of about 30 cells. Several of these cells contain large crystals. The subglandular tissue shows a well developed vascularization. In principal two cell types were found. When Sudan Black B or the selective osmium impregnation method were applied, many cells, especially those around the parenchymatous tissue and under the vascular bundles, were stained brown to black. They can be distinguished from neighboring cells by their large size and intense absorption of stains or high electron density. They deposit in their large vacuoles osmiophilic substances, presumably oil and have been named oil cells. Groups of oil droplets, in contact with the nucleus, were also present in the parenchymatous and oil cells. Tissue fixation in glutaraldehyde and osmium tetroxide vapors revealed granular electron dense remnants within oil cells. Another cell type, the mucilage cells, was found in the subglandular tissue. These cells became evident after the application of histochemical methods for polysaccharides. Autoradiographical study showed that, externally applied labelled glucose incorporated into the mucilage and cell walls. Mucilage plays an important role in the regulation of water economy, which is decisive in nectar secretion. The oil cells mainly protect the sugar containing tissues (phloem, parenchyma) against herbivores.