Isospores

Isospores are spores that are uniform in size and form, a characteristic of isosporous or homosporous plants. In this context, a single type of spore is produced by all sporangia of a sporophyte, in contrast to heterospores, where two distinct spore sizes occur (microspores and megaspores). Isospores are typically formed during meiosis within sporangia and are released to germinate into a gametophyte.

Most bryophytes (mosses, liverworts, hornworts) and the majority of ferns are homosporous, producing isospores that usually germinate into a single type of gametophyte. The resulting gametophyte is often bisexual, bearing both male and female reproductive organs, though some species may have separate sexes. Spore morphology within a species tends to be uniform, but there can be variation between species.

Evolutionarily, isospory is considered the more ancestral condition among land plants. Heterospory—the production of distinct microspores and megaspores—appears in several lineages later in plant evolution and is linked with the development of seeds in seed plants. The transition to heterospory is associated with advantages such as the production of unisexual gametophytes and greater protection for the female gametophyte.

In fungi and other kingdoms, the term isospores is less commonly used; in botany, it mainly describes the spore type characteristic of bryophytes and most ferns. The concept of isospores helps distinguish plants with a single spore type from those with differentiated, heterosporous spore production.