Treatment of acetolysed pollen grains with potassium permanganate exposes substructures by removing part of the exine. New surface details are exposed between tectal and infratectal elements. In sectioned pollen grains ectexinous components including columellae and the tectum are eventually reduced to a system of spaces delimited by a layer which is resistant to the treatment. This is referred to as a "boundary layer" and is composed of a granular or fibrillar meshwork. Results from investigations of Fagus sylvatica and Scorzonera hispanica are compared with earlier studies of exine dissolution and a new hypothesis for exine substructure is presented. In this interpretation the boundary layer is conceived as delimiting areas which are receptive to sporopollenin during early development and so define ectexine stratification and ornamentation. Subsequent addition of tapetal or microsporal sporopollenin masks the boundary layer to a greater or lesser extent in different taxa. The ability to reveal the boundary layer in mature pollen may provide ontogenetically significant information concerning the organisation of pollen walls. © 1987.
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