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. Treboux[250], in his work on organic acids, has also proved by experimental cultures with a large series of algae, including the gonidia of _Peltigera_, that these green plants in the absence of light and in pure cultures would grow and form carbohydrates if the culture medium contained a small percentage of organic acids. The acids he employed were combined with potassium and were thus rendered neutral or slightly alkaline; acetate of potash proved to be the most advantageous compound of any that was tested. Amino-acids and ammonia salts were added to provide the necessary nitrogen. Oxalic acid and other organic acids of varying composition are peculiarly abundant in lichen tissues and may be a source of carbon supply. Marshall Ward[251] has found calcium carbonate crystals in the lower air-containing tissues of _Strigula complanata_. Treboux finally concluded from his researches that just as fungi can extract carbohydrates from many sources, so algae can secure their carbon supply in a variety of ways. He affirms that the metabolic activity of the alga in these cultural conditions is entirely normal, and the various cell-contents are formed as in the light. Whether, in this case, starch is formed directly from the acids or through a series of combinations has not been determined. Uhlir[252], with electric lighting, made successful cultures of _Nostoc_ isolated from Collemaceae on silicic acid, proving thereby that these gonidia do not require a rich nutriment. A certain definite humidity was however essential, and bacteria were never eliminated as they are associated with the gelatinous membranes of Nostocaceae.