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No hyphal penetration of the bright-green algal cell by means of haustoria had been observed by the earlier workers, Bornet[225], Bonnier[226] and others, though they followed Schwendener[227] in regarding the relationship as one of host and parasite. Lindau, also, after long study accepted parasitism as the only adequate explanation of the associated growth, though he never found the fungus actually preying on the alga.

In recent years interest in the subject has been revived by the researches of Elenkin[228], a Russian botanist who claims to have established a case for parasitism or rather “endosaprophytism.” He has demonstrated by means of staining reagents the presence in the thallus of large numbers of dead algal cells. A few empty membranes are to be found in the cortex and in the gonidial zone, but the larger proportion occur below the gonidial zone and partly in the medulla. He describes the lower layer as a “necral” or “hyponecral” zone, and he considers that the hyphae draw their nourishment chiefly from dead algal material. The fungus must therefore be regarded in this case as a saprophyte rather than a parasite. The algae, he considers, may have perished from want of sufficient light and air or they may have been destroyed by an enzyme produced by the fungus. The latter he thinks is the more probable, as dead cells are frequently present among the living algae of the gonidial zone. To the action of the enzyme he also attributes the angular deformed appearance of many gonidia and the paler colour and gradual disintegration of their contents which are finally used up as endosaprophytic nourishment by the fungus. Dead algal cells were more easily seen, he tells us, in crustaceous lichens associated with “Pleurococcus” or “Cystococcus”; they were much less frequent in the larger foliose or fruticose lichens. Dead cells of Trentepohlia were also difficult to find.

In a second paper Elenkin records one clear instance of a haustorium entering an algal cell, and says he found some evidence of hyphal branches penetrating otherwise uninjured gonidia, round holes being visible in their outer wall, but he holds that it is the cell-wall of the alga that is mainly dissolved by the ferment and then used as food by the hyphae.

No allowance has been made by Elenkin for the normal wasting common to all organic beings: the lichen fungus is continually being renewed, especially in the cortical structures, and the alga must also be subject to change. He[229] claims, nevertheless, that his observations have proved that the one symbiont is always preying on the other, either as a parasite or as a saprophyte. He has likened the conception of symbiosis to that of a balance between two organisms, “a moveable equilibrium of the symbionts.” If, he says, we could conceive a state where the conditions of life would be equally favourable for both partners there would be true mutualism, but in practice one only is favoured and gains the upper hand, using its advantage to prey on the other. Unless the balance is redressed, the complete destruction of the weaker is certain, and is followed in time by the death of the stronger. The fungus being the dominant partner, the balance, he considers, is tipped in its favour.

Elenkin’s conclusions are not borne out by the long continued and healthy life of the lichen. There is no record of either symbiont having succumbed to the other, and the alga, when set free, is unchanged and able to resume its normal development. Without the alga the fungus cannot form the ascigerous fruit. Is that because as a parasite within the lichen it has degenerated past recovery, or has it become so adapted to symbiosis that in saprophytic conditions it fails to develop?

Another Russian lichenologist, U. N. Danilov[230], records results which would seem to support the theory of parasitism. He found that from the clasping hyphae minute haustoria were produced, which penetrate the algal cell-wall, and branch when within the outer membrane, thus forming a delicate network over the plasma; secondary haustoria arising from this network protrude into the interior and rob the cell-contents. He observed gonidia filled with well-developed hyphae and these, after having exhausted one cell, travel onwards to others. Some gonidia under the influence of the fungus had become deformed and were finally killed. As a proof of this latter statement he adduces the presence in the thallus of some gonidia containing shrivelled protoplasm, of others entirely empty. He considers, as further evidence in favour of parasitism, the finding of empty membranes as well as of colourless gonidia filled with the hyphal network. This description hardly tallies with the usual healthy appearance of the gonidial zone in the normal thallus, and it has been suggested that where the fungus filled the algal cell, it was as a saprophyte preying on dead material.

The gradual perishing of algal cells in time by natural decay and their subsequent absorption by the fungus is undisputed. It is open to question whether the varying results recorded by these workers have any further significance.

These observations of Elenkin and Danilov have been proved to be erroneous by Paulson and Somerville Hastings[231]. They examined the thalli of several lichens (Xanthoria parietina, Cladonia sp., etc.) collected in early spring when vegetative growth in these plants was found to be at its highest activity. They found an abundant increase of gonidia within the thallus, which they regarded as sporulation of the algae, and the most careful methods of staining failed to reveal any case of penetration of the gonidia by the hyphae.

Nienburg[232] has published some recent observations on the association of the symbionts. In the wide cortex of a Pertusaria he found not only the densely compact hyphae, but also isolated gonidia. In front of these latter there was a small hollow cavity and, behind, parallel hyphae rich in contents. These gonidia had originated from the normal gonidial zone. They were moved upward by special hyphae called by Nienburg “push-hyphae.” After their transportation, the algae at once divide and the products of division pass to a resting stage and become the centre of a new thalline growth. A somewhat similar process was noted towards the apex of Evernia furfuracea. Radial hyphae pushed up the cortex, leaving a hollow space over the gonidial zone. Into the space isolated algae were thrust by “push-hyphae.” In this lichen he also observed the penetration of the algal cell by haustoria of the fungus. He considers that the alga reaps advantage but also suffers harm, and he proposes the term helotism to express the relationship.

An instructive case of the true parasitism of a fungus on an alga has been described by Zukal[233] in the case of Endomyces scytonemata which he calls a “half-lichen.” The mature fungus formed small swellings on the filaments of the Scytonema and, when examined, the hyphae were seen to have attacked the alga, penetrating the outer gelatinous sheath and then using up the contents of the green cells. It is only after the alga has been destroyed and absorbed, that asci are formed by the fungus. Zukal contrasts the development of this fungus with the symbiotic growth of the two constituents in Ephebe where both grow together for an indefinite time.

Mere associated growth however even between a fungus and an alga does not constitute a lichen. An instance of such growth is described by Sutherland[234] in an account of marine microfungi. One of these, a species of Mycosphaerella, was found on Pelvetia canaliculata, and Sutherland claims that as no apparent injury was done to the alga, it was a case of symbiosis and that there was formed a new type of lichen. The mycelium, always intercellular, pervaded the whole host-plant, and the fungal fruits were invariably formed on the algal receptacles close to the oogonia. Their position there is, of course, due to the greater food supply at that region. Both fungus and alga fruited freely. A closer analogy could have been found by the writer in the smut fungus which grows with the host-cereal until fruiting time; or with the mycorrhiza of Calluna which also pervades every part of the host-plant without causing any injury. In the true lichen, the alga, though constituting an important part of the vegetative body, takes no part in reproduction, except by division and increase of the vegetative cells within the thallus. The fruiting bodies are always of a modified fungal nature.