An antheridium or antherida (plural: antheridia) is a haploid structure or organ producing and containing male gametes (called antherozoids or sperm). It is present in the gametophyte phase of cryptogams like bryophytes and ferns, and also in the primitive vascular psilotophytes. Many algae and some fungi, for example ascomycetes and water moulds, also have antheridia during their reproductive stages.
An antheridium typically consists of sterile cells and spermatogenous tissue. The sterile cells may form a central support structure or surround the spermatogenous tissue as a protective jacket. The spermatogenous cells give rise to spermatids via mitotic cell division. In bryophytes, the antheridium is borne on an antheridiophore, a stalk-like structure that carries the antheridium at its apex.
In many gymnosperms and all angiosperms, the male gametophytes have been reduced to pollen grains and their antheridia have been reduced to a single generative cell within the pollen grain. During pollination, this generative cell divides and gives rise to two sperm nuclei.