Frank[209] suggested the term _Homobium_ as appropriate, but it is faulty inasmuch as it expresses a relationship of complete interdependence, and it has been proved that the fungus partly, and the alga entirely, have the power of free growth.
A wider currency was given to this view of a mutually advantageous growth by de Bary[210]. He followed Reinke in refusing to accept as satisfactory the theory of simple parasitism, and adduced the evident healthy life of the algal cells—the alleged victims of the fungus—as incompatible with the parasitic condition. He proposed the happily descriptive designation of a _Symbiosis_ or conjoint life which was mostly though not always, nor in equal degree, beneficial to each of the partners or symbionts.
_b._ DIFFERENT FORMS OF ASSOCIATION. The type of association between the two symbionts varies in different lichens. Bornet[211], in describing the development of the thallus in certain members of the Collemaceae, found that though as a rule the two elements of the thallus, as in some species of _Collema_ itself, persisted intact side by side, there was in other members of the genus an occasional parasitism: short branches from the main hyphae applied themselves by their tips to some cell of the _Nostoc_ chain (Fig. 9). The cell thus seized upon began to increase in size, and the plasma became granular and gathered at the side furthest away from the point of attachment. Finally the contents were used up, and nothing was left but an empty membrane adhering to the fungus hypha. In another species the hypha penetrated the cell. These instances of parasitism are most readily seen towards the edge of the thallus where growth is more active; towards the centre the attached cells have become absorbed, and only the shortened broken chains attest their disappearance. The other cells of the chains remain uninjured.
[Illustration: Fig. 9. _Physma chalazanum_ Arn. Cells of _Nostoc_ chains penetrated and enlarged by hyphae × 950 (after Bornet).]
In _Synalissa_, a small shrubby gelatinous genus, the hypha, as described by Bornet and by Hedlund[212], pierces the outer wall of the gelatinous alga (_Gloeocapsa_) and swells inside to a somewhat globose haustorium which rests in a depression of the plasma (Fig. 10). The alga, though evidently undamaged, is excited to a division which takes place on a plane that passes through the haustorium; the two daughter-cells then separate, and in so doing free themselves from the hypha.