Fig. 78 A. Hairs of Peltigera aphthosa Willd. associated with Nostoc colony much magnified (after Babikoff).
Fig. 78 B. Peltigera aphthosa Willd. Vertical section of thallus and cephalodium × 480 (after Babikoff).
Th. M. Fries[468] had previously described the development of the cephalodia in Stereocaulon but failed to find the earliest stages. He concluded from his observations that parasitic algae were common in the cortical layer of the lichens, but only rarely formed the “monstrous growths” called cephalodia.
b. Endotrophic. Winter[469] examined the later stages of internal cephalodine formation in a species of Sticta. The alga, probably a species of Rivularia, which gives origin to the cephalodia, may be situated immediately below the upper cortex, in the medullary layer close to the gonidial zone, or between the pith and the under cortex. The protuberance caused by the increasing tissue, which also contains the invading alga, arises accordingly either on the upper or the lower surface. In some cases it was found that the normal gonidial layer had been pushed up by the protruding cephalodium and lay like a cap over the top. The cephalodia described by Winter are endogenous in origin, though the mature body finally emerges from the interior and becomes either epigenous or hypogenous. Schneider[470] has followed the development of a somewhat similar endotrophic or endogenous type in Sticta oregana due also to the presence of a species of Rivularia. How the alga attained its position in the medulla of the thallus was not observed.
Fig. 79. Nephroma expallidum Nyl. Vertical section of thallus. a, endotrophic cephalodium × 100 (after Forssell).
Both the algal cells of internal cephalodia and the hyphae in contact with them increase vigorously, and the newly formed tissue curving upwards or downwards appears on the outside as a swelling or nodule varying in size from that of a pin-head to a pea. On the upper surface the gonidial zone partly encroaches on the nodule, but the foreign alga remains in the centre of the structure well separated from the thalline gonidia by a layer of hyphae. The group is internally divided into small nests of dark-green algae surrounded by strands of hyphae ([Fig. 79]). The swellings, when they occur on the lower surface of the lichen, correspond to those of the upper in general structure, but there is no intermixture of thalline gonidia. That Nostoc cells can grow and retain the power to form chlorophyll in adverse conditions was proved by Etard and Bouilhac[471] who made a culture of the alga on artificial media in the dark, when there was formed a green pigment of chlorophyll nature.
Endotrophic cephalodia occur in many groups of lichens. Hue[472] states that he found them in twelve species of Aspicilia. As packets of blue-green algae they are a constant feature in the thallus of Solorinae. The species of that genus grow on mossy soil in damp places, and must come frequently in contact with Nostoc colonies. In Solorina crocea an interrupted band of blue-green algae lies below the normal gonidial zone and sometimes replaces it—a connecting structure between cephalodia and a true gonidial zone.