PARASITISM 261 



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 Chytri- 

 diaceae ; 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 ^ but these are not described as doing 

 damage to the host. They are mostly species of Lecidea or of allied genera. 

 Kupfer^ has also listed some 15 different lichens that he found on Lenzites sp. 



b. Antagonistic Symbiosis. In discussing the nutrition of lichens^ 

 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* example is that of Lecanora atriseda which 

 in its early stages lives. on the thallus ol Rhizocarpon geographicum inhabiting 

 mountain rocks. A detailed examination of the relationship between these 

 two plants was made by Malme and later by Bitter'. Both writers found 

 that the Lecanora thallus as it advanced caused a blackening of the Rhizo- 

 carpon 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. I^ecidea insularis {L. intumescens) 

 similarly takes possession of and destroys the thallus of Lecanora glaucoma 

 and Malme^ strongly suspects that Buellia verruculosa and B. aethalea may 

 be living on the thallus of Rhizocarpon distincturn with which they are 

 constantly associated. 



Other cases of facultative parasitism have been studied by Hofmann", 

 more especially three different species, Lecanora dispersa, Lecanora sp. and 

 Parmelia hyperopia, 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 oppor- 

 tunity offered. The thallus of the latter was in all cases distorted, the area 

 occupied by the invaders being finally killed. The attacking lichens had 



1 Arnold 1874. ^ Kupfer 1894. ^ See p. 236. . * Malme 1895. 



^ Bitter 1899. ^ Hofmann 1906. 



