PARASITISM 265 



and found that they travelled towards the gonidia and clasped them lichen- 

 wise without damaging them, since these remained green and capable of 

 division. At no stage was any harm caused to the host fay the alien 

 organism. Another instance he observed was that of Conida rubescens on 

 the thallus of Rhizocarpon epipolium. By means of fine sections through the 

 apothecia of Conida and the thallus of the host, he proved the presence of 

 numerous gonidia in the subhymenial tissue, these being closely surrounded 

 by the hyphae of the parasite, and entirely undamaged : they retained their 

 green colour, and in size and form were unchanged. Zopf 1 at first described 

 these parasites as fungi though later 1 he allows that they may represent 

 lower forms of lichens. 



Tobler 2 has added two more of these parasymbiotic species on the border 

 line between lichens and fungi, similar to those described by Zopf. One of 

 these, Phacopsis vulpina, belonging to the fungus family Celidiaceae, is 

 parasitic on Letharia vulpina. The fronds of the host plant are considerably 

 altered in form by its presence, being more branched and curly. Where 

 the parasite settles a swelling arises filled with its hyphae, and the host 

 gonidia almost disappear from the immediate neighbourhood, only a few 

 "nests" being found and these very mucilaginous. These nests as well as 

 single gonidia are surrounded by Phacopsis hyphae which have gradually 

 displaced those of the Letharia thallus. The gonidia are excited to division 

 and increase in number on contact with either lichen or fungus hyphae, but 

 in the latter case the increase is more abundant owing doubtless to a more 

 powerful chemical irritant in the fungus. As development advances, the 

 Phacopsis hyphae multiply to the exclusion of both lichen hyphae and 

 gonidia from the area of invasion. Finally the host cortex is split, the 

 fungus bursts through, and the tissue beneath the parasite becomes brown 

 and dead. Phacopsis begins as a "parasymbiont," then becomes parasitic, 

 and is at last saprophytic on the dead cells. The hyphae travel down into 

 the medulla of the host and also into the soredial outgrowths, and are 

 dispersed along with the host. The effect of Verrucula on the host thallus 

 may also be cited 8 . 



Tobler gives the results of his examination of still another fungus, Kar- 

 schia destructans. It becomes established on the thallus of Chaenotheca 

 chrysocephala and its hyphae gradually penetrate down to the underlying 

 bark (larch). The lichen thallus beneath the fungus is killed, but gonidia in 

 the vicinity are sometimes clasped : Karschia also is thus a parasymbiont, 

 then a parasite, and finally a saprophyte. 



Elenkin 4 describes certain fungi which to some extent are parasymbionts. 

 One of these, Conidella urceolata n.sp., grew on forms of Lecanora esculenta. 

 The other, a stroma-forming species, had invaded the thallus of Parmelia 

 1 Zopf 1898, p. 249. 2 Tobler 191 1 2 . 3 See p. 276. 4 Elenkin i9oi 2 . 



