Microchannels in the Pollen Grain Exine

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Abstract
Direct evidence for the location of microchannels within larger diameter structures in modern pollen grain exines is provided by selective oxidation of the exine bordering the microchannels. These large structures form a three‐dimensional network that can be seen throughout exines in all transmission electron microscope studies involving experimental degradation of the exine in extant taxa and also in some fossil exines. From our results we conclude that components forming this reticulated pattern must be aligned around channel‐containing structures and that the superpositioning of the network of loops can be expected to be mostly radial in orientation, like the microchannels. Our approach was to oxidize the exines of Betula verrucosa and Chenopodium album after first causing the exines either to expand or contract. In expanded exines microchannels were etched out, creating cylindrical cavities 40–50 nm in diameter and the entire exine became highly acceptable to staining. Compaction of exines restricted the tunneling of oxidant into the exine more or less to microchannels. MicroChannel zones became 40–70 nm wide and strongly stainable while the rest of the exine remained only weakly stainable. Penetration of stain throughout expanded but not contracted exines indicates the opening or closure of microcapillary space within structures of the exines.