The number of such fungi determined and classified has gradually increased, and now extends to a very long list. Even as far back as 1896 Zopf reckoned up 800 instances of parasitism of 400 species of fungi on about 350 different lichens and many more have been added. Abbé Vouaux[960] is the latest writer on the subject, but his work is mostly a compilation of species already known. He finds representatives of these parasites in nine families of Pyrenomycetes and six of Discomycetes. He leaves out of account the much debated Coniocarps, but he includes with fungi all those that have been proved to be parasymbiotic, such as Abrothallus.
A number of fungus genera, such as Conida, etc., are parasitic only on lichens. Most of them have one host only; others, such as Tichothecium pygmaeum, live on a number of different thalli. Crustaceous species are often selected by the parasites, and no great damage, if any, is caused to these hosts, except when the fungus is seated on the disc of the apothecium, so that the spore-bearing capacity is lessened or destroyed.
In some of the larger lichens, however, harmful effects are more visible. In Lobaria pulmonaria, the fruits of which are attacked by the Discomycete, Celidium Stictarum[961], there is at first induced an increased and unusual formation of lichen apothecia. These apothecia are normally seated for the most part on the margins of the lobes or pustules, but when they are invaded by the fungus, they appear also in the hollows between the pustules and even on the under surface of the thallus. In the large majority of cases the fungus is partly or entirely embedded in the thallus; the gonidia in the vicinity may remain green and healthy, or all the tissues in the immediate neighbourhood of the parasite may be killed.
f. Mycetozoa Parasitic on Lichens. Mycetozoa live mostly on decayed wood, leaves, humus, etc. One minute species, Listerella paradoxa, always inhabits the podetia of Cladonia rangiferina. Another species, Hymenobolina parasitica, was first detected and described by Zukal[962] as a true parasite on the thallus of Physciaceae; it has since been recorded in the British Islands on Parmeliae[963]. This peculiar organism differs from other mycetozoa in that the spores on germination produce amoebae. These unite to form a rose-red plasmodium which slowly burrows into the lichen thallus and feeds on the living hyphae. It is a minute species, but when abundant the plasmodia can just be detected with the naked eye as rosy specks scattered over the surface of the lichen. Later the grey sporangia are produced on the same areas.
F. Diseases of Lichens
a. Caused by Parasitism. Zopf[964] has stated that of all plants, lichens are the most subject to disease, reckoning as diseases all the instances of parasitism by fungi or by other lichens. There are however only rare instances in which total destruction or indeed any permanent harm to the host is the result of such parasitism. At worst the trouble is localized and does not affect the organism as a whole. Some of these cases have been already noted under antagonistic symbiosis or parasymbiosis. Several instances have however been recorded where real injury has been caused by the penetration of some undetermined fungus mycelium. Zukal[965] records two such observed by him in Parmelia encausta and Physcia villosa: the thallus of the former was dwarfed and deformed by the presence of the alien mycelium, the latter was excited to abnormal proliferation.
b. Caused by crowding. Lichens suffer frequently from being overgrown by other lichens; they may also be crowded out by other plants. My attention was called by Mr P. Thompson to a burnt plot of ground in Epping Forest, which, after the fire, had been colonized by Peltigera spuria. In the course of a few years, other vegetation had followed, depriving the lichen of space and light and gradually driving it out. When last examined only a few miserable specimens remained, and these were reduced in vitality by an attack of the lichen parasite Illosporium carneum.
c. Caused by adverse conditions. Zukal considers as pathological, at least in origin, the cracking of the thallus so frequent in crustaceous lichens as well as in the more highly developed forms. As the cracks are beneficial in the aeration of the plant, they can hardly be regarded as symptoms of a diseased condition. The more evident ringed breaks in the cortex of Usneae, due probably to wind action, have more reason to be so regarded; they are most pronounced in Usnea articulata, where the portions bounded by the rings are contracted and swollen, and a hollow space is formed between the cortex and the central axis. The swellings that are produced on lichen thalli, such as those of Umbilicaria and some species of Gyrophora, due to intercalary growth are normal to the plant, though occasionally the swollen weaker portions may become ruptured and the cortex be thrown off. As pathological also must be regarded the loss of cortex sometimes occasioned by excessive soredial formation at the margins of the lobes: the upper cortex may be rolled back and eventually torn away; the gonidial layer is exposed and transformed into soredia which are swept away by the wind and rain, till finally only traces of the lower cortex are left.
Zukal[966] has instanced, as a case of diseased condition observed by him, the undue thickening of the cortex in Pertusaria communis whereby the formation of the fruiting bodies is inhibited and even vegetative development is rendered impossible. There arrives finally a stage when splitting takes place and the whole thallus breaks down and disappears. As a rule however there need be no limit to the age of the lichen plant. There is no vital point or area in the thallus; injury of one part leaves the rest unhurt, and any fragment in growing condition, if it combines both symbionts, can carry on the life of the plant, the constant renewal of gonidia preventing either decay or death. Barring accidents many lichens might exist as long as the world endures.