The evolution of miospore polarity

Abstract

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 dista...