Influence of Pinocytosis and Membrane Specializations on Pollen Wall Form

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Abstract
Three features of developing pollen grains may facilitate the uptake of materials: (1) tubules (∼ 10 nm in diameter) at the surface of the exine, (2) wall tubules (7–18 nm in diameter), and (3) channels through the exine (irregular in shape, but generally between 50 and 300 nm in diameter). The tubules of Aegiceras occur in tufts on the surface of the exine and extend from the exine to the tapetal cell surface. A glycocalyx (1,2-glycols and acidic groups) is associated with the tubules, and we suggest that the tubules and glycocalyx may be a specialization of a unit membrane on the exine surface. The wall tubules of Nuphar are found in the outer part of the exine and extend from the nexine-2 to the tectum, including the tips of spines. Evaginations of the plasma membrane penetrate the exine of young microspores of Epilobium and their trails remain as channels in the mature exine. We show that lanthanum nitrate can pass directly through the substance, not only the channels, of the exine in microspores of Epilobium.