In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoans. A chief difference between spores and seeds as dispersal units is that spores have very little stored food resources compared with seeds.
Spores are usually haploid and unicellular and are produced by meiosis in the sporangium by the sporophyte. Once conditions are favorable, the spore can develop into a new organism using mitotic division, producing a multicellular gametophyte, which eventually goes on to produce gametes.
Two gametes fuse to create a new sporophyte. This cycle is known as alternation of generations, but a better term is "biological life cycle", as there may be more than one phase and so it cannot be a direct alternation. Haploid spores produced by mitosis (known as mitospores) are used by many fungi for asexual reproduction.
Many ferns, especially those adapted to dry conditions, produce diploid spores. This form of asexual reproduction is called apogamy. It is a form of apomixis.
Spores are the units of asexual reproduction, because a single spore develops into a new organism. By contrast, gametes are the units of sexual reproduction, as two gametes need to fuse to create a new organism.