Spore

Spore, term applied to a specially modified asexual reproductive cell produced by many fungi and plants and by some protozoans. The spore is resistant to heat, drought, and other adverse conditions, remaining in a resting state until the environment is favorable for development or germination. Many bacteria (see Prokaryote) concentrate their cytoplasm and encapsulate under unfavorable conditions; these resting stages are generally called spores, but they are not reproductive cells and are therefore not comparable to the spores of other organisms.

Spores are usually produced by the division of cells within a structure called a sporangium. In bryophytes and most ferns, horsetails, club mosses, and whisk ferns, spores give rise to the same kind of plants as the parents, which are thus called homosporous. But in a few of those just mentioned and in all seed plants, some spores grow into sexual plants (gametophytes) that produce male gametes (sex cells), while others grow into sexual plants that produce female gametes; parent plants producing such spores are called heterosporous. Spores that give rise to male gametophytes are called microspores; spores giving rise to female gametophytes are known as megaspores. In seed plants, male gametophytes are known as pollen grains, and female gametophytes are called embryo sacs. Thick-walled resting zygotes (cells formed by the union of gametes) resemble spores and are called zygospores or oospores.