Examples of algal association with other plants might be cited here: of _Nostoc_ in the roots of _Cycas_ and in the cells of _Anthoceros_, and of _Anabæna_ in the leaf-cells of _Azolla_, but in these instances it is generally held that the alga secures only shelter. It was by comparing the lichen-association with the harmless invasion of _Gunnera_ cells by _Nostoc_ that Reinke[243] arrived at his conception of “consortism.”
_d._ SUPPLY OF CARBON. Carbon, the essential constituent of all organic life, is partly drawn from the carbon-dioxide of the air, and assimilated by the green cells; it is also partly contributed by the fungus as a product of its metabolism. A proof of this is afforded by Dufrenoy[244]: he found a _Parmelia_ growing closely round pine needles and even sending suckers into the stomata. He covered the lichen with a black cloth and after seven weeks found that the gonidia had remained very green. That growth had not been checked was evidenced by an unusual development of soredia and of spermogonia. Dufrenoy describes the condition as a parasitism of the algae on the fungus which in turn was drawing nourishment from the pine needles.
Artari[245] has proved that lichen gonidia can obtain carbohydrates from the substratum as well as by photosynthesis. He cultivated the gonidia of _Xanthoria parietina_ and _Placodium murorum_ on media which contained organic substances as well as mineral salts, while depriving them of atmospheric carbon-dioxide and in some cases of light also. The gonidia not only grew well but, even in the dark, they remained normally green, a phenomenon coinciding with Etard and Bouilhac’s[246] experience in growing _Nostoc_ in the dark: with suitable culture media the alga retained its colour. _Nostoc_ also grows in the dark in the rhizome of _Gunnera_. Radais’[247] experiments with _Chlorella vulgaris_ confirmed these results. On certain organic media growth and cell-division were as rapid in the dark as in the light, and chlorophyll was formed. The colour was at first yellowish and the full green arrived slowly, especially on sugar media, but in ten days it was uniform and normal.
When making further experiments with the alga, _Stichococcus bacillaris_, Artari[248] found that it also grew well on an organic medium and that grape sugar was the most valuable carbonaceous food supply. Chodat[249] also found that sugar or glucose was a desirable ingredient of culture media.