Prothallus

A prothallus, or prothallium, (from Latin pro = forwards and Greek θαλλος (thallos) = twig) is usually the gametophyte stage in the life of a fern or other pteridophyte.

It is a short-lived and inconspicuous heart-shaped structure typically 2–5 millimeters wide, with a number of rhizoids (root-like hairs) growing underneath, and the sex organs: archegonium (female) and antheridium (male).

The fully grown sporophyte, what is commonly referred to as the fern, produces genetically unique spores in the sori by meiosis.

The prothallus develops independently for several weeks; it grows sex organs that produce ova (archegonia) and flagellated sperm (antheridia).

[2] In the large genus Cladonia, the prothallus may provide a mode of vegetative reproduction, and it may have a role in stabilising the soil.