One of the exceedingly minute spores found in certain flowerless plants, as Selaginella and Isoetes, which bear two kinds of spores, one very much smaller than the other. Cf. Macrospore.
The smaller of two kinds of spore borne by a plant, usually giving rise to a male prothallium ; pollen-grain.
In seed plants, it gives rise to the pollen grain, the male gametophyte.