Abstract
The proexine that forms within the callosic envelope before the end of the microspore tetrad period is thick (about 1 μm) and exceptionally complex. It has components equatable with tectum, columellae, and a nexine that includes lamellar zones. All these components persist in the exine although late in development they become difficult to recognize because this exine is reduced in thickness, apparently by stretching, to a maximum of 0.2 μm. Strelitzia is an example of an exine template, with receptors for sporopollenin, that is not maintained during development. The Strelitzia microspore surface changes from an exine like that on an interaperture sector to the channeled intinelike system common for the apertures of pollen grains. The exine on sterile grains gives what may be a rare view of a stabilized immature exine. The mature exine on viable pollen grains resembles this early exine only in the most impressionistic way. Tapetal cells go through at least one cycle of hyperactivity, dedifferentiation, mitosis, and then again hyperactivity before they finally decline.
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