PARASITISM 267 



observed. The mature fruiting body had no distinct excipulum, but was 

 surrounded by a layer of dead lichen cells. 



It is not easy to determine the difference between parasites that are of 

 fungal nature and those that are lichenoid ; but as a general rule the fungi 

 may be recognized by their more transient character, veiy frequently by 

 their effect on the host thallus, which is more harmful than that produced 

 by lichens, and generally by their affinity to fungi rather than to lichens. 

 Opinions differ and will continue to differ on this very difficult question. 



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. Abb6 Vouaux' 

 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 Tichotheciuiii 

 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\ there is at first induced an increased and unusual forma- 

 tion 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. 



/ 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' as a 

 true parasite on the thallus of Physciaceae; it has since been recorded in the 

 British Islands on Parmeliae'^. 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 



' Vouaux ipii, etc. ^ Bitter 1904. ^ Zukal 1893. 'i Lister 1911. 



