Spores can be divided into three groups according to mode of origin and place in the developmental cycle of the plant: (1) diploid zygotes, (2) mitospores, and (3) haploid meiospores. 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). 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). 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. They are usually somewhat spherical or ellipsoidal in shape cylindrical or other forms occur occasionally. Spores are unicellular or, less commonly, bicellular or multicellular structures. (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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