Botany online 1996-2004. No further update, only historical document of botanical science!
Cross-Sections of the Pollen Wall of
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a. Cross-section through a rib of the tectum as seen through a scanning electron microscope: the broad and cylindrical columellae merge into the foot-layer.b. A section through the perforation layer that shows the extremely narrow apertures at the exine's surface (V. BITTRICH, 1986).