PARASITISM 263 



there on the thallus of Umbilicariae or between the areolae of Buelliae, and 

 always too vigorous to be ousted from its position. 



Bacidia flavovirescens has been regarded by some lichenologists 1 as a 

 parasite on Baeomyces, but recent work by Tobler 2 seems to have proved 

 that the bright green thallus is that of the Bacidia. 



c. PARASYMBIOSIS. There are certain lichens that are obligative parasites 

 and pass their whole existence on an alien thallus. They may possibly have 

 degenerated from the condition of facultative parasitism as the universal 

 history of parasitism is one of increased dependence on the host, and of 

 growing atrophy of the parasite, but, in the case of lichens, there is always 

 the peculiar symbiotic condition to be considered : the parasite produces its 

 own vigorous hyphae and normal healthy fruits, it often claims only a share 

 of the carbohydrates manufactured by the gonidia. The host lichen is not 

 destroyed by this parasymbiosis though the tissues are very often excited 

 to abnormal growth by the presence of the invading organism. 



Lauder Lindsay 3 was one of the first to study these "microlichens" as 

 he called them, and he published descriptions of those he had himself 

 observed on various hosts. He failed however to discriminate between lichens 

 and parasitic fungi. It is only by careful research in each case that the 

 affinity to fungi or to lichens can be determined; very frequently the whole 

 of them, as possessing no visible thallus, have been classified with fungi, but 

 that view ignores the symbiosis that exists between the hyphae of the 

 parasite and the gonidia of the host. 



Parasitic lichens are rather rare on gelatinous thalli ; but even among 

 these, a few instances have been recorded. Winter 4 has described a species 

 of Leptoraphis, the perithecia of which are immersed in the thallus of Physma 

 franconicum. The host is wholly unaffected by the presence of the parasite 

 except for a swelling where it is situated. The foreign hyphae are easily 

 distinguishable; they wander through the thallus of the host with their free 

 ends in the mucilage of the gonidial groups from which they evidently 

 extract nourishment. Species of the lichen genus Obryzum are also parasitic 

 on gelatinous lichens. 



The parasitic genus Abrotkallus* has been the subject of frequent study. 

 There are a number of species which occur as little black discs on various 

 thalli of the large foliose lichens. They were first of all described as parasitic 

 fungi, later Tulasne 6 affirmed their lichenoid nature as proved by the struc- 

 ture, consistence and long duration of the apothecia. Lindsay 7 wrote a 

 monograph of the genus dealing chiefly with Abrothallus Smithii (Buellia 

 Parmeliarum) and A. oxysporus, with their varieties and forms that occur on 



1 Th. Fries 1874, p. 343. 2 Tobler 191 1 2 . 3 Lindsay 18692. 4 Winter 1877. 



6 Abrothallus has been included in the lichen genus Buellia. 6 Tulasne 1852. 



7 Lindsay 1856. 



