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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).

An instance of epiphytic growth has also been recorded by Zahlbruckner[310]. He found an alga, Trentepohlia abietina, covering the thallus of a Brazilian lichen, Parmelia isidiophora, and growing so profusely as to obscure the isidiose character towards the centre of the thallus. There was no genetic connection of the alga with the lichen as the former was not that of the lichen gonidium. Lichen thalli are indeed very frequently the habitat of green algae, though their occurrence may be and probably is accidental.