Fink: Classification of Lichens 
J37 
garcling the relation of the lichen parasite to its algal host. Thus 
it comes about that many botanists have thought the relation to be 
mutualistic, some even going so far as to suppose that the host and 
the parasite together constitute an individual, working in harmony 
much like the parts of an ordinary automaton. But the facts do 
not favor this supposition. The host is placed in a disadvanta- 
geous position regarding food supply, and then more or less of its 
food is probably carried to it from the substratum with which it 
would be in direct contact, were it not for being parasitized by the 
lichen, which imprisons it and removes it from the substratum. 
We see nothing in this, on the whole, but disadvantage to the algal 
host, which could secure its food more easily, were it not parasi- 
tized. 
Arnoldia and Physina are the only lichen genera parasitic on 
Nostoc, which send haustoria into the cells of this algal host, or 
even come into close enough relation with these cells to effect any 
change in their form or that of the filaments which they compose. 
Otherwise, the cells or the filaments of the algal hosts of lichens 
are more or less modified by the parasitism. Yet one may find, 
commonly enough, algal cells or filaments not yet parasitized, 
which show the normal form for the species. In the case of 
Nostoc, where the cells are not usually affected, the whole colony 
is nevertheless modified in form by the parasitic lichen, as is 
illustrated by species of the Collemaceae. 
Bonnier (31) germinated spores of Physcia apiolia (Ach.) Nyl. 
on the protonemata of mosses and found that the hyphae invest 
the protonemata, form a plectenchyma and haustoria and sap 
the protonemata of nourishment. Those on which the lichens 
grow are smaller than those not parasitized, and are often killed 
outright, though the haustoria do not penetrate into the cells of 
the protonemata. He found also that the same often occurs in 
Cladonia pyxidata (L.) Hoffm., when the spores germinate on 
protonemata and grow as above explained. Sometimes algae 
fall upon the lichen growing on the protonemata, when the algae 
become parasitized, and the lichen finally reaches its full develop- 
ment. This indicates that the relation of the lichen to the alga 
is parasitism, as is its relation to the moss protonemata ; but the 
algae are not so easily killed, and the lichen develops fully when 
it grows with them. 
