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;