Fern (Dryopteris filix-mas) prothallus. Light micrograph of a section through the prothallus of a 'male fern', showing the sex organs: the male antheridia (purple) and female archegonia (pink). 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. Magnification: x6 when printed 10 centimetres wide