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Many unicellular plants and animals reproduce both by the formation of spores and by simple cell division ( mitosis mitosis The term reproduction may refer to this power of self-duplication of a single cell or a multicellular animal or plant organism. , process of nuclear division in a living cell by which the carriers of hereditary information, or the chromosomes, are exactly replicated and the two copies distributed to identical daughter nuclei. Yeasts, for instance, reproduce by forming spores as well as by budding. In multicellular plants the sporophyte generation produces (by meiosis meiosis Among the fungi some spores are thin-walled and germinate quickly others are thick-walled resistant types. , process of nuclear division in a living cell by which the number of chromosomes is reduced to half the original number. Click the link for more information.) spores with half the normal number of chromosomes chromosome Meiosis occurs only in the process of gametogenesis, i.e., when the gametes, or sex cells (ovum and sperm), are being formed.
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, structural carrier of hereditary characteristics, found in the nucleus of every cell and so named for its readiness to absorb dyes. Click the link for more information. 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. (1) In lower and higher plants, a microscopic rudiment of varying origin that facilitates reproduction and/or survival under unfavorable conditions. Spores are unicellular or, less commonly, bicellular or multicellular structures. They are usually somewhat spherical or ellipsoidal in shape cylindrical or other forms occur occasionally. The spores of many plants have tough, frequently sculptured, sheaths of complex structure such spores usually retain their germinating power a long time, because their protoplasts contain reserve nutrients. The spores of lower plants are often given names that reflect their structural characteristics (actively motile flagellated zoospores and nonflagellated aplanospores), their shape (stylo-spores), or their capacity to grow (auxospores). Their names may denote the presence of a thick sheath (chlamydospores), the type of fruiting body in which the spores are produced (sporangio-spores, carpospores, ascospores, basidiospores, and aecio-spores), origin in a spore case or on a sporogenous organ (endo-spores and exospores), the number of spores in a single sporangium (monospores and tetraspores), the type of sexual process resulting in sporogenesis (oospores and zygospores), or the category of plants to which the spores belong (smut spores and urediospores).