E. Non-gonidial Organisms associated with Lichen Hyphae
Bonnier[307] made a series of cultures with lichen spores and green cells other than those that form lichen gonidia. In one instance he substituted Protococcus botryoides for the normal gonidia of Parmelia (Xanthoria) parietina; in another of his cultures he replaced Protococcus viridis by the filamentous alga Trentepohlia abietina. In both cases the hyphae attached themselves to the green cells and a certain stage of thallus formation was reached, though growth ceased fairly early. Another experiment made with the large filaments of Vaucheria sessilis met with the same amount of success ([Fig. 34]). The germinating hyphae attached themselves to the alga and grew all round it, but there was no advance to tissue formation.
Cultures were also made with the protonema of mosses. Either spores of mosses and lichens were germinated together, or lichen spores were sown in close proximity to fully formed protonemata. The developing hyphae seized on the moss cells and formed a network of branching anastomosing filaments along the whole length of the protonema without, however, penetrating the cells. If suitable algae were encountered, proper thallus formation commenced, and Bonnier considers that the hyphae receive stimulus and nourishment from the protonema sufficient to tide them over a considerable period, perhaps until the algal symbiont is met. An interesting variation was noted in connection with the cultures of Mnium hornum[308]. If the protonema were of the usual vigorous type, the whole length was encased by the hyphal network; but if it were delicate and slender, the protoplasm collected in the cell that was touched by hyphae and formed a sort of swollen thick-walled bud ([Fig. 35]). This new body persisted when the rest of the filament and the hyphae had disappeared, and, in favourable conditions, grew again to form a moss plant.
Fig. 34. Germinating hyphae of Lecanora subfusca Ach., growing over the alga Vaucheria sessilis DC., much magnified (after Bonnier).
F. Parasitism of Algae on Lichens
A curious instance of undoubted parasitism by an alga, not as in Strigula on one of the higher plants, but on a lichen thallus, is recorded by Forssell[309]. A group of Protococcus-like cells established on the thallus of Peltigera had found their way into the tissue, the underlying cortical cells having degenerated. The blue-green cells of the normal gonidial layer had died off before their advance but no zone was formed by the invading algae; they simply withdrew nourishment and gave seemingly no return. The phenomenon is somewhat isolated and accidental but illustrates the capacity of the alga to absorb food supply from lichen hyphae.
Fig. 35. Pure culture of protonema of Mnium hornum L. with spores and hyphae of Lecidea vernalis Ach. a,a,a, buds forming × 150 (after Bonnier).