Wallroth introduced the term ‘gonidia’ to describe the green cells found in lichens. They were considered to be ‘brood cells’ or organs of reproduction of the lichen-fungus. This interpretation of their function was accepted until the appearance of the classic work of Schwendener2. Although it was to be many years before the theory of the dual nature of lichens, proposed by Schwendener, was universally accepted, the gradual realization of the identity of the gonidia with free-living algae, brought about a complete change in meaning of this term. No longer could it retain its original application to cells which were thought to take part in the reproduction of the fungal component, but came to be accepted as a term applied to an organism, capable of independent existence, and in no way connected with reproduction of the lichen-fungus. Its continued use has been fostered by convenience at the expense of terminological exactitude.