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.