When the gametophyte has grown to its adult size, it produces the sex organs of the hornwort. Most plants are monoecious, with both sex organs on the same plant, but some plants (even within the same species) are dioecious, with separate male and female gametophytes. The female organs are known as archegonia (singular archegonium) and the male organs are known as antheridia (singular antheridium). Both kinds of organs develop just below the surface of the plant and are only later exposed by disintegration of the overlying cells.