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. 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. Upon germination, spores that were generated asexually may produce cells or multicellular forms that can engage in sexual reproduction. Many unicellular plants and animals reproduce both by the formation of spores and by simple cell division (mitosis). Yeasts, for instance, reproduce by forming spores as well as by budding. Among the fungi some spores are thin-walled and germinate quickly; others are thick-walled resistant types. 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.

Reproductive cell capable of developing into a new individual without fusing with another reproductive cell. Spores thus differ from gametes, which must fuse in pairs in order to create a new individual. Spores are agents of nonsexual reproduction; gametes are agents of sexual reproduction. Spores are produced by bacteria, fungi (see fungus), and green plants. Bacterial spores serve largely as a resting, or dormant, stage in the life cycle, preserving the bacterium through periods of unfavorable conditions. Many bacterial spores are highly durable and can germinate even after years of dormancy. Fungal spores serve a function similar to that of seeds in plants; they germinate and grow into new individuals under suitable conditions of moisture, temperature, and food availability. Among green plants (all of which have a life cycle characterized by alternation of generations), spores are the reproductive agents of the nonsexual generation (sporophyte), giving rise to the sexual generation (gametophyte).

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