STRUCTURES PECULIAR TO LICHENS 133 



2. CEPHALODIA 



A. Historical and Descriptive 



The term " cephalodium " was first used by Acharius' to designate cer- 

 tain 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 " cor- 

 puscula," " maculae," etc. The term is now restricted to those purely vege- 

 tative 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^ 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^ within the thallus of Aspt'd/m app. 



Florke^ in his account of German lichens described the cephalodia that 

 grow on the podetia of Stereocaulon as fungoid bodies, ''corpuscula fungosa." 

 Wallroth^, 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^ in his 

 Monograph of Stereocaulon and Pilophorus; but the greatest advance in 

 the exact knowledge of these bodies is due to Forssell" who made a com- 

 prehensive 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 some- 

 times very rare even when present, and then difficult to find. A striking 

 instance of variability in their occurrence is recorded for Ricasolia amplis- 

 sima {Lobaria laciniata) (Fig. jG). The cephalodia of that species are 

 prominent upright branching structures which grow in crowded tufts irregu- 

 larly 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. 



^ Acharius 1803. ^ Hue 1904 and 1910. ^ Florke 1815, IV. p. 15. 



* Wallroth 1825, p. 678. ^ Th. M. Fries 1858. " Forssell 1884. 



