(1) In lower and higher plants, a microscopic rudiment of varying origin that facilitates reproduction and/or survival under unfavorable conditions. In multicellular plants the sporophyte generation produces (by meiosis) spores with half the normal number of chromosomes for the species these grow directly into the gametophyte generation, which produces (by mitosis) male and female reproductive cells that when united give rise to a sporophyte. Among the fungi some spores are thin-walled and germinate quickly others are thick-walled resistant types. Yeasts, for instance, reproduce by forming spores as well as by budding. Many unicellular plants and animals reproduce both by the formation of spores and by simple cell division ( mitosis). Upon germination, spores that were generated asexually may produce cells or multicellular forms that can engage in sexual reproduction. A spore is typically a cell surrounded by a cell wall in resistant spores and in the resting stage of reproductive spores this wall becomes tough and waterproof, permitting the cell to survive unfavorable circumstances such as extremes of temperature and moisture. Spore, term applied both to a resistant or resting stage occurring among various unicellular organisms (especially bacteria) and to an asexual reproductive cell produced by many unicellular plants and animals and by all plants that undergo an alternation of generations.
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