266 BIONOMICS 



moUiuscula, where it caused gall-formation. As the growth of the gall was 

 due to the co-operation of the lichen gonidia, the fungus must at first have 

 been a parasymbiont. Only dead gonidia were present in the stroma; prob- 

 ably they had been digested by the parasite. Because of the stroma Elenkin 

 placed the fungus in a new genus, Trematosphaeriopsis. 



e. Fungi Parasitic on Lichens. A solution or extract of lichen 

 thallus is a very advantageous medium in which to grow fungi. It is there- 

 fore not surprising that lichens are a favourite habitat for parasitic fungi. 

 StahU has noted that the lichens themselves flourish best where there is 

 frequent moistening by rain or dew with equally frequent drying which 

 effectively prevents the growth of fungi. Species of Peltigera are hpwever 

 able to live in damp conditions : without being injured, they have been 

 observed to maintain their vigour when cultivated in a very moist hot- 

 house while all the other forms experimented with were attacked and finally 

 destroyed by various fungi. 



Lindsay^ devoted a great deal of attention to the microscopic study of 

 the minute fruiting bodies so frequently present on lichen thalli and published 

 descriptions of microlichens, microfungi and spermogonia. He and others 

 naturally considered these parasitic organisms to be in many cases either 

 the spermogonia or pycnidia of the lichen itself It is often not easy to 

 determine their relationship or their exact systematic position ; many of 

 them are still doubtful forms. 



There exists however a very large number of fully recognized parasitic 

 microfungi belonging to various genera. Lindsay discovered many of them. 

 Zopf has given exact descriptions of a series of forms, with special reference 

 to their effect on the host thallus. In an early paper he described a species, 

 Pleospora collematum, that he found on Physma compacium and other Colle- 

 maceae. The hyphae of the parasite differed from those of the host in being 

 of a yellow colour ; they did not penetrate or spread far, being restricted to 

 rhizoid-like filaments at the base of their fruiting bodies (perithecia and 

 pycnidia). Their presence caused a slight protuberance but otherwise did 

 no harm to the host ; the Nostoc cells in their immediate vicinity were even 

 more brightly coloured than in other parts of the thallus. In another paper' 

 he gives an instance of gall-formation in Collema pulposum induced by the 

 presence of the fungus Didymosphaeria pulposi. Small protuberances were 

 formed on the margins of the apothecia, more rarely on the lobes of the 

 thallus, each one the seat of a perithecium of the fungus. No damage was 

 done to either constituent of the thallus. 



Agyrium flavescens grows parasitically on the under surface of Peltigera 

 polydactyla. M. and Mme Moreau^ found that the hyphae of the fungus 

 spread between the medullary filaments of the lichen; no haustoria were 



1 Stahl 1904. ^ Lindsay 1859, 1869, 1871. ^ Zopf 1896. ^ Zopf 1898. « Moreau 1916'. 



