128 
]\Iycologia 
obtained from the air. But now, through another work of 
Tobler’s (132), reinforced by some other researches, it appears 
reasonable to suppose that the lichen may furnish the alga a por- 
tion of the carbohydrates which the former receives from the 
substratum. Tobler cultivated Xanthoria parietina (L.) Th. Fr. 
and some other lichens without algae in a beerwort-gelatine 
medium and got a rich production of calcium oxalate. He then 
grew the thalli with the algae and was not able to find the oxalate. 
Therefore, he supposes that the algae use the surplus extracted 
from the medium by the lichen, and that the same thing may 
occur in nature, the lichen taking the oxalate from the organic 
substratum. Tobler also started Xanthoria on gelatine and trans- 
ferred it to a licjuid medium which contained none of the carbon 
compounds needed by the alga, except what was contained in the 
air, and the lichen grew somewhat. ‘ The alga increased in the 
same liquid medium and was of normal appearance. In trans- 
ferring the lichen, particles of the gelatine were unavoidably 
carried over. After these were probably consumed by the lichen, 
the alga was introduced into the culture with the lichen and grew 
well, but was colorless. This he thinks indicates that the lichen 
had assimilated the acid, probably oxalic, which the alga needed 
as a source of carbon. He says that gelatine is not a source of 
carbon for the alga, so that his conclusions would not be invali- 
dated even if the lichen had not extracted all of this substance 
from the medium before the alga was introduced. The lichen 
hyphae-soon entwine some of the algal cells in the culture, and 
thus the parasitic relationship is established. Tobler believes 
that the lichen obtains carbon from the alga, while the latter 
replaces it by extracting carbon from the oxalic acid contained in 
the tissues of the lichen. This would mean an exchange of food 
particles between the lichen and its algal host in the cultures. 
While this may be true in his cultures, it could scarcely be so in 
instances where the lichen grows on rocks that contain no organic 
matter. But organic substances accumulate very soon, on or in 
all exposed rocks ; so all rocks on which lichens grow may have 
sufficient organic matter. However, there is no certainty, if 
indeed any probability, that the physiological relation of the lichen 
