Forssell[997] gave considerable attention to the question of antiquity in his study of gelatinous crustaceous lichens in the family Pyrenopsidaceae, termed by him Gloeolichens, and he came to the conclusion that Archilichens represented the older combination, Phycolichens being comparatively young.
His view is based on a study of the development of certain lichen fungi that seem able to adapt themselves to either kind of algal symbiont. He found[997] in Euopsis (Pyrenopsis) granatina, one of the Pyrenopsidaceae, that certain portions of the thallus contained blue-green algae, while others contained Palmella, and that these latter, though retrograde in development, might become fertile. The granules with blue-green gonidia were stronger, more healthy and capable of displacing those with Palmella, but not of bearing apothecia, though spermogonia were embedded in them—a first step, according to Forssell, towards the formation of apothecia. These granules, not having reached a fruiting stage, were reckoned to be of a more recent type than those associated with Palmella. In other instances, however, the line of evolution has been undoubtedly from blue-green to more highly evolved bright-green thalli.
The striking case of similarity between Psoroma hypnorum (bright-green) and Pannaria rubiginosa (blue-green) may also be adduced. Forssell considers that Psoroma is the more ancient form, but as the fungus is adapted to associate with either kind of alga, the type of squamules forming the thallus may be gradually transformed by the substitution of blue-green for the earlier bright-green—the Pannaria superseding the Psoroma. There is a close resemblance in the fructification—that is of the fungus—in these two different lichens.
Hue[998] shares Forssell’s opinion as to the greater antiquity of the bright-green gonidia and cites the case of Solorina crocea. In that lichen there is a layer of bright-green gonidia in the usual dorsiventral position, below the upper cortex. Below this zone there is a second formed entirely of blue-green cells. Hue proved by his study of development in Solorina that the bright-green were the normal gonidia of the thallus, and were the only ones present in the growing peripheral areas; the blue-green were a later addition, and appeared first in small groups at some distance from the edge of the lobes.
The whole subject of cephalodia-development[999] has a bearing on this question. These bodies always contain blue-green algae, and are always associated with Archilichens. Mostly they occur as excrescences, as in Stereocaulon and in Peltigera. The fungus of the host-lichen though normally adapted to bright-green algae has the added capacity of forming later a symbiosis with the blue-green. This tendency generally pervades a whole genus or family, the members of which, as in Peltigeraceae, are too closely related to allow as a rule of separate classification even when the algae are totally distinct.
C. Evolution of Phycolichens
The association of lichen-forming fungi with blue-green algae may have taken place later in time, or may have been less successful than with the bright-green: they are fewer in number, and the blue-green type of thallus is less highly evolved, though examples of very considerable development are to be found in such genera as Peltigera, Sticta or Nephromium.
a. Gloeolichens. Among crustaceous forms the thallus is generally elementary, more especially in the Gloeolichens (Pyrenopsidaceae). The algae of that family, Gloeocapsa, Xanthocapsa or Chroococcus, are furnished with broad gelatinous sheaths which, in the lichenoid state, are penetrated and traversed by the fungal filaments, a branch hypha generally touching with its tip the algal cell-wall. Under the influence of symbiosis, the algal masses become firmer and more compact, without much alteration in form; algae entirely free from hyphae are often intermingled with the others. Even among Gloeolichens there are signs of advancing development both in the internal structure and in outward form. Lobes free from the substratum, though very minute, appear in the genus Paulia, the single species of which comes from Polynesia. Much larger lobes are characteristic of Thyrea, a Mediterranean and American genus. The fruticose type, with upright fronds of minute size, also appears in our native genus Synalissa. It is still more marked in the coralloid thalli of Peccania and Phleopeccania. In most of these genera there is also a distinct tendency to differentiation of tissues, with the gonidia congregating towards the better lighted surfaces. The only cortex formation occurs in the crustaceous genus Forssellia in which, according to Zahlbruckner[1000], it is plectenchymatous above, the thallus being attached below by hyphae penetrating the substratum. In another genus, Anema[1001], which is minutely lobate-crustaceous, the internal hyphae form a cellular network in which the algae are immeshed. As regards algal symbionts, the members of this family are polyphyletic in origin.
b. Ephebaceae and Collemaceae. In Ephebaceae the algae are tufted and filamentous, Scytonema, Stigonema or Rivularia, the trichomes of which are surrounded by a common gelatinous sheath. The hyphae travel in the sheath alongside the cell-rows, and the symbiotic plant retains the tufted form of the alga as in Lichina with Rivularia, Leptogidium with Scytonema, and Ephebe with Stigonema. The last named lichen forms a tangle of intricate branching filaments about an inch or more in length. The fruticose habit in these plants is an algal characteristic; it has not been acquired as a result of symbiosis, and does not signify any advance in evolution.