Pollen development in Epilobium (Onagraceae): From microspore mitosis to formation of the intine

Development of the pollen grains of Epilobium angustifolium L. from microspore mitosis until formation of the intine with special attention to development of the apertures. In the period before and during intine production the aperture regions extend dramatically, considerably beyond their extent in mature pollen grains. This extension is greatly reduced after intine development by virtue of the great enlargement of the body of the pollen grain. During early formation of the endexine components (tufts) that extend across the aperture vestibule consist as a core zone ca 40 nm in diameter and a surrounding binder zone that is 10 - 15 nm in diameter. Toward the end of aperture development the core remains about 40 nm in diameter but the surrounding binder becomes more than 500 nm in diameter, showing that growth of binder components of exine-units are responsible for and increase in size and mass of the exine. Results with a tracer show that some nutrients and other substances can pass through the exine in living pollen grains at a late stage of development. In Epilobium the exine is essential for transport into the cytoplasm since apertures are covered by exine until late in development.