_b._ SUPPLY OF NITROGEN. Important contributions on the subject of algal nutrition have been made by Beyerinck[235] and Artari[236]. The former conducted a series of culture experiments with green algae, including the gonidia of _Physcia_ (_Xanthoria_) _parietina_. He successfully isolated the lichen gonidia and, at first, attempted to grow them on gelatine with an infusion of the Elm bark from which he had taken the lichen. Growth was very slow and very feeble until he added to the culture-medium a solution of malt-extract which contains peptones and sugar. Very soon he obtained an active development of the gonidia, and they multiplied rapidly by division[237] as in the lichen thallus. This proved to him conclusively the great advantage to the algae of an abundant supply of nitrogen. Artari in his work has demonstrated that there are two different physiological races of green algae: (1) those that absorb peptones—which he designates peptone-algae—and (2) those that do not so absorb peptones. He tested the cells of _Cystococcus humicola_ taken from the thallus of _Physcia parietina_, and found that they belonged to the peptone group and were therefore dependent on a sufficiency of nitrogenous material to attain their normal vigorous growth. It was also discovered by Artari that the one race can be made by cultivation to pass over to the other: that ordinary algae can be educated to live on peptones, and peptone-algae to do without. We learn further from Beyerinck’s researches that Ascomycetes, the group of fungi from which the hyphae of most lichens are derived, are what he terms ammonia-sugar fungi; that is to say, the hyphae can abstract nitrogen from ammonia salts and, with the addition of sugar, can form peptones. The lichen peptone-algae are thus evidently, by their contact with such fungi, in a favourable position for securing the nitrogenous food supply most suited to their requirements. In their deep-seated layers, they are to a large extent deprived of light, but it has been proved by Artari[238] in a series of culture experiments extending over a long period, that the gonidia of _Xanthoria parietina_ remain green in the dark under very varied conditions of nutriment, though the colour is distinctly fainter.