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a. Consortium and Symbiosis. These cultures had established convincingly the composite nature of the lichen thallus, and Schwendener’s opinion, that the relationship between the two organisms was some varying degree of parasitism, was at first unhesitatingly accepted by most botanists. Reinke[206] was the first to point out the insufficiency of this view to explain the long continued healthy life of both constituents, a condition so different from all known instances of the disturbing or fatal parasitism of one individual on another. He recognized in the association a state of mutual growth and interdependence, that had resulted in the production of an entirely new type of plant, and he suggested Consortium as a truer description of the connection between the fungus and the alga. This term had originally been coined by his friend Grisebach in a paper[206] describing the presence of actively growing Nostoc algae in healthy Gunnera stems; and Reinke compared that apparently harmless association with the similar phenomenon in the lichen thallus. The comparison was emphasized by him in a later paper[207] on the same subject, in which he ascribes to each “consort” its function in the composite plant, and declares that if such a mutual life of Alga and Ascomycete is to be regarded as one of parasitism, it must be considered as reciprocal parasitism; and he insists that “much more appropriate for this form of organic life is the conception and title of Consortium.” In a special work on lichens, Reinke[208] further elaborated his theory of the physiological activity and mutual service of the two organisms forming the consortium.

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.

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.

Fig. 10. Synalissa symphorea Nyl. Algae (Gloeocapsa) with hyphae from the internal thallus × 480 (after Bornet).

Hedlund followed the process of association between the two organisms in the lichens Micarea (Biatorina) prasina and M. denigrata (Biatorina synothea), crustaceous species which inhabit trunks of trees or palings. In these the alga, one of the Chlorophyceae, has assumed the character of a Gloeocapsa but on cultivation it was found to belong to the genus Gloeocystis. The cells are globose and rather small; they increase by the division of the contents into two or at most four portions which become rounded off and covered with a membrane before they become free from the mother-cell. The lichen hypha, on contact with any one of the green cells, bores through the outer membrane and swells within to a haustorium, as in the gonidia of Synalissa.

Fig. 11. Gonidia from Ramalina reticulata Nyl. A, gonidium pierced and cell contents shrinking × 560; B, older stage, the contents of gonidium exhausted × 900 (after Peirce).

Fig. 12. Pertusaria globulifera Nyl. Fungus and gonidia from gonidial zone × 500 (after Darbishire).

Penetrating haustoria were demonstrated by Peirce[213] in his study of the gonidia of Ramalina reticulata. In the first stage the tip of a hypha had pierced the outer wall of the alga, causing the protoplasm to contract away from the point of contact (Fig. 11). More advanced stages showed the extension of the haustorium into the centre of the cell, and, finally, the complete disappearance of the contents. In many cases it was found that penetration equally with clasping of the alga by the filament sets up an irritation which induces cell-division, and the alga, as in Synalissa, thus becomes free from the fungus. Hue[214] has recorded instances of penetration in an Antarctic species, Physcia puncticulata. It is easy, he says, to see the tips of the hyphae pierce the sheath of the gonidium and penetrate to the nucleus.

Lindau[215] has described the association between fungus and alga in Pertusaria and other crustaceous forms as one of contact only (Fig. 12). He found that the cell-membrane of the two adhering organisms was unbroken. Occasionally the algal cell showed a slight indentation, but was otherwise unchanged. The hyphal branch was somewhat swollen at the tip where it touched the alga, and the wall was slightly thinner. The attachment between the two cells was so close, however, that pressure on the cover-glass failed to separate them.

Generally the hypha simply surrounds the gonidium with clasping branches. Many algae also lie free in the gonidial zone, and Peirce[216] claims that these are larger, more deeply coloured and in every way healthier looking than those in the grasp of the fungus. He ignores, however, the case of the soredial algae which though very closely invested by the fungus are yet entirely healthy, since on their future increase depends in many cases the reproduction of new individual lichens.

In a recent study of a crustaceous sandstone lichen, “Caloplaca pyracea,” Claassen[217] has sought to prove a case of pure parasitism. The rock was at first covered with the green cells of Cystococcus sp. Later there appeared greyish-white patches on the green, representing the invasion of the lichen fungus. These patches increased centrifugally, leaving in time a bare patch in the centre of growth which was again colonized by the green alga. The lichen fruited abundantly, but wherever it encroached the green cells were more or less destroyed. The true explanation seems to be that the green cells were absorbed into the lichen thallus, though enough of them persisted to start new colonies on any bare piece of the stone. In the same way large patches of Trentepohlia aurea have been observed to be gradually invaded by the dark coloured hyphae of Coenogonium ebeneum. In time the whole of the alga is absorbed and nothing is to be seen but the dark felted lichen. The free alga as such disappears, but it is hardly correct to describe the process as one of destruction.

This algal genus Trentepohlia (Chroolepus) forms the gonidia of the Graphideae, Roccelleae, etc. It is a filamentous aerial alga which increases by apical growth. In the Graphideae, many of which grow on trees beneath the outer bark (hypophloeodal), the association between the two symbionts may be of the simplest character, but was considered by Frank[218] to be of an advanced type. According to his observations and to those of Lindau[219], the fungal hyphae penetrate first between the cells of the periderm. The alga, frequently Trentepohlia umbrina, tends to grow down into any cracks of the surface. It goes more deeply in when preceded by the hyphae. In some species both organisms maintain their independent growth, though each shows increased vigour when it comes into contact with the other. In some instances the cells of the alga are clasped by the fungus which causes the disintegration of the filament. The cells lose their bright yellow or reddish colour and are changed in appearance to greenish lichen gonidia; but no penetration by haustoria has ever been observed in Trentepohlia.

Bachmann’s[220] study of a similar gonidium in a calcicolous species of Opegrapha confirms Frank’s results. The algae had pierced not only between the looser lime granules but also through a crystal of calcium carbonate, and occupied nests scooped out in the rock by means of acid formed and excreted by their filaments. When association took place with the fungus, the algal cells were more restricted to a gonidial zone; but some of the cells, having been pushed aside by the hyphae, had started new centres of gonidia. On contact with the hyphae there was a tendency to bud out in a yeast-like growth.

In the thallus of the Roccelleae, the algal filament, also a Trentepohlia, is broken up into separate cells, but in the Coenogoniaceae, whether the gonidium be a Cladophora as in Racodium, or a Trentepohlia as in Coenogonium, the filaments remain intact and are invested more or less closely by the hyphae.