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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 by 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
at first described
green colour, and in size and form were unchanged.
1
Zopf
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 3 .
Tobler gives the results of his examination of still another fungus, Kar-
schia destructans. It becomes established on the thallus of Chaenotheca
cJnysoceptiala 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, Conidclla 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 1901-.