Their quick recovery seems also a strong argument in favour of the absolutely normal condition of metabolism within the gonidial cell; and the paler appearance of the chlorophyll is doubtless associated with the acquisition of carbohydrates from other sources than by photosynthesis. There is a wide difference between any degree of unfavourable life-conditions and parasitism however slight, even though the balance of gain is on the side of the fungus. It is not too fanciful to conclude that the demand for nitrogen on the part of the alga has influenced its peculiar association with the fungus. In the thallus of hypophloeodal lichens it has been proved indeed that the alga _Trentepohlia_ with apical growth is an active agent in the symbiotic union. _Cystococcus_ and other green algal cells are stationary, but they are doubtless equally ready for—as many of them are equally benefited by—the association. Keeble[242] has pointed out in the case of _Convoluta roscoffensis_ that nitrogen-hunger induces the green algae to combine forces with an animal organism, though the benefit to them is only temporary and though they are finally sacrificed. The lichen gonidia, on the contrary, persist for a long time, probably far beyond their normal period of existence as free algae. 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.