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. [Illustration: 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. [Illustration: 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. CHAPTER III MORPHOLOGY GENERAL ACCOUNT OF LICHEN STRUCTURE I. ORIGIN OF LICHEN STRUCTURES