Fink: Classification of Lichens 
129 
to its algal host is, in nature, like what occurs in cultures. It is 
more probable, after all, that the lichen simply carries from the 
substratum whatever food it gives to the algal host. Yet the 
lichen may assimilate, or at least digest, some of this food before 
it is used by the algal host. Tobler obtained the same results 
with Pertusaria communis Lam. & DC. and Parmelia acetabulum 
(Neck.) Dub. So the conclusion is that the lichen produces the 
oxalate in the cultures, and that the lichen consumes it in each 
instance. ^lultiplying instances of such action in cultures can 
not give us certain knowledge of what happens in nature. 
The only certain thing shown by these experiments is that the 
lichen stores up substances when growing alone in cultures, which 
are not stored when it grows with the alga, since this host plant uses 
these substances as a source of carbon. In Pertusaria and Diplo- 
schistes, Tobler found oxalic acid cystals present in thalli which 
contained algal host cells. He supposes that, in these and other 
lichens that have thin cortices, the algal host secures sufficient 
carbon from the air and does not utilize that secured by the 
lichen from the substratum. In lichens that have thick cortices, 
the alga would have greater difficulty in securing carbon from the 
air and would be more likely to depend in part, or perhaps wholly, 
on materials taken from the substratum by the lichen. 
The Growth of Lichen Hosts and Other Algae in Pure 
Cultures 
Treboux (133) found that species of Chlorella, Pleurococcus, 
Cystococcus and some other algae are able to obtain carbon in a 
very different manner from that known in higher green plants. 
These algae are able to thrive on artificial media containing 
organic acids, and so he reached the conclusion that they behave like 
fungi with respect to carbon assimilation. Artari (7) grew 
Chlorococcum humicola, obtained from the thalli of Xanthoria 
parietina (L.) Th. Fr. and Placodium murorum (Hoffm.) Ach., 
in pure cultures on complicated media containing mineral salts or 
organic compounds, or in some instances, both the mineral salts and 
the organic compounds. He found that the alga grows and multi- 
plies luxuriantly on the media containing organic matter and is dark 
green in color, though grown in absolute darkness or in light with 
