There are however some lichens belonging to widely diverse genera that have retained, or reverted to, the saprophytic or parasitic habit of their fungal ancestors, though the cases that occur are generally of lichens preying on other lichens. The conditions have been described as those of “antagonistic symbiosis” when one lichen is hurtful or fatal in its action on the other, and as “parasymbiosis” when the association does little or no injury to the host. The parasitism of fungi on lichens, though falling under a different category, in many instances exhibits features akin to parasymbiosis.
The parasitism of fungus on fungus is not unusual; there are instances of its occurrence in all the different classes. In the Phycomycetes there are genera wholly parasitic on other fungi such as Woronina and other Chytridiaceae; Piptocephalus, one of the Mucorini, is another instance. Cicinnobolus, one of the Sphaeropsideae, preys on Perisporiae; a species of Cordyceps is found on Elaphomyces, and Orbilia coccinella on Polyporus; while among Basidiomycetes, Nyctalis, an agaric, grows always on Russula.
There are few instances of lichens finding a foothold on fungi, for the simple reason that the latter are too short lived. On the perennial Polyporeae a few have been recorded by Arnold[929], but these are not described as doing damage to the host. They are mostly species of Lecidea or of allied genera. Kupfer[930] has also listed some 15 different lichens that he found on Lenzites sp.
b. Antagonistic Symbiosis. In discussing the nutrition of lichens[931] note has been taken of the extent to which some species by means of enzymes destroy the thallus of other lichens in their vicinity and then prey on the dead tissues. A constantly cited[932] example is that of Lecanora atriseda which in its early stages lives on the thallus of Rhizocarpon geographicum inhabiting mountain rocks. A detailed examination of the relationship between these two plants was made by Malme and later by Bitter[933]. Both writers found that the Lecanora thallus as it advanced caused a blackening of the Rhizocarpon areolae, the tissues of which were killed by the burrowing slender filaments of the Lecanora, easily recognized by their longer cells. The invader thereafter gradually formed its own medulla, gonidial layer and cortex right over the surface of the destroyed thallus. Lecidea insularis (L. intumescens) similarly takes possession of and destroys the thallus of Lecanora glaucoma and Malme[932] strongly suspects that Buellia verruculosa and B. aethalea may be living on the thallus of Rhizocarpon distinctum with which they are constantly associated.
Other cases of facultative parasitism have been studied by Hofmann[934], more especially three different species, Lecanora dispersa, Lecanora sp. and Parmelia hyperopta, which were found growing on the thick foliose thallus of Dermatocarpon miniatum. These grew, at first independently, on a wall along with many examples of Endocarpon on to which they spread as opportunity offered. The thallus of the latter was in all cases distorted, the area occupied by the invaders being finally killed. The attacking lichens had benefited materially by the more nutritive substratum: their apothecia were more abundant and their thallus more luxuriant. The gonidia especially had profited; they were larger, more brightly coloured, and they increased more freely. Hoffmann offers the explanation that the strain on the algae of providing organic food for the hyphal symbiont was relaxed for the time, hence their more vigorous appearance.
Arthonia subvarians is always parasitic on the apothecia of Lecanora galactina, and Almquist[935] discovered that the hymenium of the host alone is injured, the hypothecium and excipulum being left intact.
The “parasitism” of Pertusaria globulifera on Parmelia perlata and P. physodes, as described by Bitter[936], may also be included under antagonistic symbiosis. The hyphae pierce the Parmelia thallus, break it up and gradually absorb it. Chemical as well as mechanical influences are concerned in the work of destruction as both the fungus and the alga of the victim are dissolved. Lecanora tartarea already dealt with as a marauding lichen[937] over decaying vegetation may spread also to living lichens. Fruticose soil species, such as Cetraria aculeata and others, die from the base and the Lecanora gains entrance to their tissues at the decaying end which is open.
Arnold[938] speaks of these facultative parasites that have merely changed their substratum as pseudo-parasites, and he gives a list of instances of such change. In many cases it is rather the older thalli that are taken possession of, and, in nearly every case, the invader is some crustaceous species. The plants attacked are generally ground lichens or more particularly those that inhabit damp localities, such as Peltigera or Cladonia or certain bark lichens. Drifting soredia or particles of a lichen would easily take hold of the host thallus and develop in suitable conditions. To give a few of the instances observed, there have been found, by Arnold, Crombie and others:
on Peltigera canina: Callopisma cerina, Rinodina turfacea var., Bilimbia obscurata and Lecanora aurella;
on Peltigera aphthosa: Lecidea decolorans;