Spores are diaspores (dispersal units) of fungi, ferns, fern allies, and some other plants, but they also can be resting stages in the life cycles of some animals and bacteria. Spores can be formed sexually or asexually, and many different kinds of spores exist. The chief difference between spores and seeds is that spores have very little stored food resources compared with seeds, and thus require more favorable conditions in order to germinate successfully. In compensation, spores are very hardy, and many can survive years in dry conditions.
In the case of spore-shedding vascular plants such as ferns, wind distribution of very light spores provides great capacity for dispersal. Also, spores are less subject to animal predation than seeds because they contain almost no food reserve, however they are more subject to fungal and bacterial predation. Their chief advantage is that, of all forms of progeny, spores require the least energy and materials to produce.
Vascular plant spores are always haploid and vascular plants are either homosporous or heterosporous. Plants that are homosporous produce spores of the same size and type. Heterosporous plants, such as spikemosses, quillworts, and some aquatic ferns produce spores of two different sizes: the larger spore effectively functioning as a female spore and the smaller functionally male.
See also: sporangium.