The evolution of miospore polarity

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Abstract
The evolutionary sequence from homospory through heterospory to the seed habit is reviewed in terms of the “small spore” (miospore) part of the life cycle. In the early stages of this progression, as seen in the Devonian and Carboniferous, the miospore shows little outward modification. The earliest miospores to function as pollen grains still retained proximal germination via a triradiate suture. The term “prepollen” is redefined so as to cover all such cases in which the miospore behaved as a pollen grain (in that its corresponding megaspore was retained in a ovule) but in which the germinal aperture was still proximal. The change from proximal to distal germination, which evidently lagged behind the retention of the megaspore to form an ovule, occurred independently in the cordaites and pteridosperms. The behavior of the pollen in living cycads shows an intermediate state between that of prepollen and the true pollen of the living conifers; for although the germination of cycad pollen is distal, the eventual release of the sperm is proximal. The change from the (distally) monocolpate to the ubiquitous triaperturate pollen in the angiosperms took place during the Cretaceous. One possible route of derivation is represented by the trichotomocolpate aperture present in the pollen of a few living angiosperms. The behavior of this type of pollen at germination shows that it may function as though having either one distal or three equatorial apertures.

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