Mosses are the most common, diverse, and advanced group of Bryophytes, a division of green, seedless plants that dates back to the Permian period (286 to 245 million years ago). In Bryophytes, the antheridium is the female sex organ, which produces eggs. The archegonium, illustrated along with the antheridium in the fluorescence photomicrograph presented below, is the male reproductive organ, which produces sperm.

The antheridia and archegonia of mosses are generally found at the tips of the main plant shoots. In some species, the shoots are unisexual, each shoot containing either archegonia or antheridia. Other species have bisexual shoots, containing both archegonia and antheridia.

Like other plants, most mosses reproduce through the alternation of generations, alternating generations of sexual and asexual forms; each complete life cycle requiring two generations. The sexual form, called the gametophyte (producing haploid gametes), begins as a protonema. The protonema is the direct product of spore germination and gives rise to the green leafy plant commonly regarded as a moss. The asexual form, or sporophyte (producing diploid spores), develops as separate male and female plants, but neither can exist independently. Each sporophyte plant is composed of a capsule, which is the center of spore formation; a stalk; and a foot that attaches the sporophyte body to the tip of the gametophyte. Eventually, the diploid spores are released and, upon successful germination, grow into another moss plant.