Fink: Classification of Lichens 
147 
biosis or mutualism.” The views of Bessey and Clements seem 
identical, though couched in somewhat different terms. Clements’ 
definitions of parasitism and symbiosis, like Bessey’s statement, 
are unusual, but both admit of the use of the term, parasitism, in 
designating the relation of the lichen to its algal host, even by 
those who believe in mutualism of the extreme form known as 
individualism. But parasitism and mutualism, as usually defined, 
can not be said to exist at the same time between two plants in 
symbiotic relation. The symbiotic relation must be antagonistic, 
as in parasitism, or mutualistic, as in mutualism. We admit that 
the algal host may be benefited in some ways by the association 
with the lichen; but since the injury received is, as a whole, 
greater than the possible benefit, the relation is parasitism, or 
antagonistic symbiosis. 
The Relation of the Lichen to the Substr.\tum 
Some lichens grow on certain kinds of rocks and others on 
other rocks; some on barks of certain trees and others on barks 
of other trees ; some on decorticate wood still intact and others on 
rotten wood; and some on one kind of soil and others on other 
kinds. Hence, it is reasonable to suppose that lichens secure 
some nourishment from their substrata rather than to think that 
their distribution upon various substrata is governed entirely by 
physical structure of these substrata. Moreover, we can not 
understand the relation of the lichen to its algal host without con- 
sidering the relation to the substratum ; hence, a summary of 
knowledge upon this subject must be given in this paper. 
Schwendener (117) in his epoch-making studies of the thalli of 
lichens paid little attention to the lichen rhizoids, but reached the 
general conclusion that they do not, in higher lichens, penetrate 
into the periderm to any considerable extent. Lotsy (84) studied 
the rhizoids of a number of foliose lichens and found them spread 
out on the substratum ; he thought none of them penetrated below 
the surface. Frank (58) and Bonnier (31) decided that lichen 
hyphae and Trentepohlia filaments are able to penetrate into peri- 
• derm cells and extract food from the walls and the cell contents. 
Lindau (81) found that these earlier workers were mistaken, and 
that the algae, and the lichen hyphae of hypophloeodal lichens, 
