The male, sperm-producing gametangium on the gametophytes of liverworts, mosses, hornworts, lycophytes, ferns, and horsetails, and some algae. True antheridia, with differentiated layers of sterile tissue surrounding the spermatogenous cells, first appeared in the Charales.
An antheridium (plural: antheridia) is a structure or organ of the gametophyte phase of certain plants producing and containing the spermatids or male gametes (called antherozoids). It is present in lower plants like mosses and ferns, and also in the primitive vascular psilotophytes, but the comparable structure in gymnosperms (microsporangium) and angiosperms (anthers) is comprised mostly of sporophyte (diploid) tissue that surrounds the haploid gametes. The female counterpart to the antheridium is the archegonium.