Developing fern prothallus. Light micrograph of a section through the prothallus (bottom, heart-shaped) of a 'male fern' (Dryopteris filix-mas) developing into a young sporophyte (top, leaf-like). The prothallus is the gametophyte stage in the life-cycle of a fern. It develops from a germinating spore and usually consists of a number of rhizoids (root-like hairs) and the sex organs: male antheridia and female archegonia. The sporophyte is the fully-grown spore-producing stage of the fern. It grows out of the prothallus and develops into a new independent fern. Magnification: x6 when printed 10 centimetres wide