C. ALGAE THAT FORM CEPHALODIA The algae of the cephalodia belong mostly to genera that form the normal gonidia of other lichens. They are: _Stigonema_,—in _Lecanora gelida_, _Stereocaulon_, _Pilophorus robustus_, and _Lecidea pelobotrya_. _Scytonema_,—a rare constituent of cephalodia. _Nostoc_,—the most frequent gonidium of cephalodia. It occurs in those of the genera _Sticta_, _Lobaria_, _Peltigera_, _Nephroma_, _Solorina_ and _Psoroma_; occasionally in _Stereocaulon_ and in _Lecidea pallida_. _Lyngbya_ and _Rivularia_,—rarely present, the latter in _Sticta oregana_[462]. _Chroococcus_ and _Gloeocapsa_,—also very rare. _Scytonema_, _Chroococcus_, _Gloeocapsa_ and _Lyngbya_ are generally found in combination with some other cephalodia-building alga, though Nylander[463] found _Scytonema_ alone in the lobulate cephalodia of _Sphaerophorus stereocauloides_, a New Zealand lichen, and the only species of that genus in which cephalodia are developed; and Hue[460] records _Gloeocapsa_ as forming internal cephalodia in two species of _Aspicilia_. Bornet[464] found _Lyngbya_ associated with _Scytonema_ in the cephalodia of _Stereocaulon ramulosum_, and, in the same lichen, Forssell[465] found, in the several cephalodia of one specimen, _Nostoc_, _Scytonema_, and _Lyngbya_, while, in those of another, _Scytonema_ and _Stigonema_ were present. In the latter instance these algae were living free on the podetium. Forssell[465] also determined two different algae, _Gloeocapsa magma_ and _Chroococcus turgidus_, present in a cephalodium on _Lecidea panaeola_ var. _elegans_. As a general rule only one kind of alga enters into the formation of the cephalodia of any species or genus. A form of _Nostoc_, for instance, is invariably the gonidial constituent of these bodies in the genera, _Lobaria_, _Sticta_, etc. In other lichens different blue-green algae, as noted above, may occupy the cephalodia even on the same specimen. Forssell finds alternative algae occurring in the cephalodia of: _Lecanora gelida_ and _Lecidea illita_ contain either _Stigonema_ or _Nostoc_; _Lecidea panaeola_, with _Gloeocapsa_, _Stigonema_ or _Chroococcus_; _Lecidea pelobotrya_, with _Stigonema_ or _Nostoc_; _Pilophorus robustus_, with _Gloeocapsa_, _Stigonema_, or _Nostoc_. [Illustration: Fig. 77. _Lecanora gelida_ Ach. _a_, lobate cephalodia × 12 (after Zopf).] Riddle[466] has employed cephalodia with their enclosed algae as diagnostic characters in the genus _Stereocaulon_. When the alga is _Stigonema_, as in _S. paschale_, etc., the cephalodia are generally very conspicuous, grey in colour, spherical, wrinkled or folded, though sometimes black and fibrillose (_S. denudatum_). Those containing _Nostoc_ are, on the contrary, minute and are coloured verdigris-green (_S. tomentosum_ and _S. alpinum_).