FINK—THE LICHENS OF MINNESOTA, 33 
of lichens, and one quite as much as the other is in need of further 
elucidation. 
ECONOMIC ROLE OF LICHENS. 
Lichens are among the most widely distributed of plants, and the 
traveler will find them wherever he may go. To the far north the 
species seem to decrease somewhat in number, but in the arctic 
and subarctic regions some of the plants reach considerable size and 
serve for food, both for man and for lower animals. Lichens are com- 
mon in tropical regions also; but here they have not been so fre- 
quently resorted to for food. These plants have been used in the 
arts and for medicine, and they also play an important réle in na- 
ture as purifiers of the air and in the reduction of rocks to soil. 
AS PURIFIERS OF THE AIR. 
Lichens take large amounts of carbon dioxide from the air, in the 
process of nutrition build up lichenin, a carbon compound very simi- 
lar to starch, and return to the atmosphere as free oxygen the por- 
tion not needed in the production of lichenin and other compounds 
within the plants. It was formerly supposed that the lichens took 
a very small portion, if any, of their food from the substratum, but 
that view is certainly incorrect, at least in the case of many species. 
However, it is quite likely that most lichens take a smaller part of 
their food from the substratum and a larger part from the air than 
do the higher chlorophyll-bearing plants. But it is certain that the 
fungus forming the lichen takes more or less of crude or elaborated 
food materials from the substratum, while the algal cells of the part- 
nership do the work of building up the lichenin. Thus lichens, in 
the ordinary processes of nutrition, aid in purifying the air by ex- 
tracting carbon dioxide and giving back to the air a portion of the 
oxygen in the free condition. Lichens are very sensitive to condi- 
tions of the atmosphere and are becoming scarce near our cities and 
larger towns. This is due partly to the disturbance of substrata, 
but it is also true that the dust and the impurities of the air about 
cities are In some way unfavorable to the lichens. Doubtless the 
dust fills the pores of the thallus and interferes with the passage of 
gases, while some impurities interfere with nutrition and respiration. 
It is not the intention to give the impression that lichens are the 
great conservators of atmospheric purity, but rather that they con- 
tribute their share of work toward this end. 
AS AIDS IN ROCK DISINTEGRATION. 
It is well known that certain crustose lichens are the first plants 
to attack rocks and that they aid greatly in the reduction of rocks 
