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a small spore of heterosporous Pteridophyta (Selaginella, Isoetes, Salvinia, and other water ferns) and seed plants. Microspores are usually formed in large numbers in special organs, the microsporangia, as a result of meiosis of arche-spores. Consequently, microspores are haploid. The microspore is covered with a thin inner membrane (inline) and a thicker outer membrane (exine). When the microspore of a pteridophyte sprouts (usually in the microsporangium), it forms a greatly reduced male prothallium with sex organs, the antheridia. The sprouted microspores (prothallia) are transported by water, wind, or other agents, to the female prothallia, where the spermatozoids, freed from the antheridia, penetrate the archegonia for fertilization.
In seed plants the pollen grain, which is homologous to the male prothallium, arises from the microspore in the microsporangium. In gymnosperms the pollen grain consists of several vegetative cells and one antheridial cell and forms the male gametes (in sago palms and ginkgoes there are flagellate spermatozoids; the rest of the gymnosperms have nonmotile spermatozoids). Angiosperms have the most reduced male prothallia, consisting of one vegetative and one generative cell. After the pollen falls on the stigma of the pistil, the vegetative cell becomes extended into the pollen tube and the generative cell divides, forming two spermatozoids, of which one merges with the egg cell and the other merges with the secondary nucleus of the embryo sac.
WORKSKomarnitskii, N. A., L. V. Kudriashov, and A. A. Uranov. Sistematika rastenii. Moscow, 1962.
L. V. KUDRIASHOV