The term “cephalodium” was first used by Acharius[450] to designate certain globose apothecia (pycnidia). At a later date he applied it to the peculiar outgrowths that grow on the thallus of _Peltigera aphthosa_, already described by earlier writers, along with other similar structures, as “corpuscula,” “maculae,” etc. The term is now restricted to those purely vegetative gall-like growths which are in organic connection with the thallus of the lichen, but which contain one or more algae of a different type from the one present in the gonidial zone. They are mostly rather small structures, and they take various forms according to the lichen species on which they occur. They are only found on thalli in which the gonidia are bright-green algae (Chlorophyceae) and, with a few exceptions, they contain only blue-green (Myxophyceae). Cephalodia with bright-green algae were found by Hue[451] on two _Parmeliae_ from Chili, in addition to the usual blue-green forms; the one contained _Urococcus_, the other _Gloeocystis_. Several with both types of algae were detected also by Hue[451] within the thallus of _Aspicilia_ spp. Flörke[452] in his account of German lichens described the cephalodia that grow on the podetia of _Stereocaulon_ as fungoid bodies, “corpuscula fungosa.” Wallroth[453], who had made a special study of lichen gonidia, finally established that the distinguishing feature of the cephalodia was their gonidia which differed in colour from those of the normal gonidial zone. He considered that the outgrowths were a result of changes that had arisen in the epidermal tissues of the lichens, and, to avoid using a name of mixed import such as “cephalodia,” he proposed a new designation, calling them “phymata” or warts. Further descriptions of cephalodia were given by Th. M. Fries[454] in his _Monograph of Stereocaulon and Pilophorus_; but the greatest advance in the exact knowledge of these bodies is due to Forssell[455] who made a comprehensive examination of the various types, examples of which occurred, he found, in connection with about 100 different lichens. Though fairly constant for the different species, they are not universally so, and are sometimes very rare even when present, and then difficult to find. A striking instance of variability in their occurrence is recorded for _Ricasolia amplissima_ (_Lobaria laciniata_) (Fig. 76). The cephalodia of that species are prominent upright branching structures which grow in crowded tufts irregularly scattered over the surface. They are an unfailing and conspicuous specific character of the lichens in Europe, but are entirely wanting in North American specimens. [Illustration: Fig. 76. _Ricasolia amplissima_ de Not. (_Lobaria laciniata_ Wain.) on oak, reduced. The dark patches are tufts of branching cephalodia (A. Wilson, _Photo._).]