84 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
to soil. It would seem from superficial observation that some lich- 
ens begin to grow on perfectly firm rock and, gaining a foothold, 
reach their full size and produce fruit while the rock is still in a firm 
and wholly undisintegrated condition. For instance, on the very 
hard Sioux quartzite in southern Minnesota the lichens are growing 
on perfectly smooth surfaces supposed to have been polished by the 
wind near the close of glacial times. Yet this rock shows to the 
eye or lens no evidence of disintegration and is, macroscopically, in 
exactly the same condition under the lichens as elsewhere. But in 
spite of this it is not supposed that the rhizoids of the lichens ever 
penetrate perfectly firm rock, but rather that the plant gains a point 
of attachment, perhaps in microscopic openings, and then begins to 
secrete an acid which slowly disintegrates the rock, the rhizoids pene- 
trating deeper and deeper as the work of the acid makes a way for 
them. 
In other portions of Minnesota may be found crustose lichens 
growing on rocks that have not yet fallen to fragments but are so 
rotten for several inches below the surface that they can easily be 
powdered by the foot. The work of rock disintegration is aided in 
its early stages by the lichens, and especially by the crustose forms, 
As the rock at the surface is gradually reduced to small fragments 
and soil the crustose lichens decay and add their quota of humus. 
On this bit of prepared earth, in some crevice or on a flat surface of 
rock, the foliose and fruticose lichens and certain mosses begin to 
appear and carry on the work begun by the crustose lichens. Then 
in turn appear ferns, herbaceous seed plants, and finally shrubs and 
trees, first in the crevices and at length over the whole surface, until 
the lichens are largely replaced by larger vegetation. 
AS FOOD. 
It is doubtful whether even the wild animals eat lichens to any 
considerable extent so far south as the pineries of northern Minnesota, 
for the reason that there is too much other available food. But it 
would not be surprising if some careful observations in winter would 
show that the moose, caribou, and deer eat the ‘‘reindeer moss,”’ 
Cladonia rangiferina (pl. 12, facing p. 111), and other large Cladonias 
to some extent. Farther northward the reindeer moss and some 
other lichens are important as food for both man and lower animals. 
Cladonia rangiferina and two or three closely related species form 
the principal food of the reindeer and become in Lapland relatively 
as important as some of the grasses of our prairies. This happens 
for the reason that the larger plants do not drive out the lichens so 
effectually in regions to the north, so that the reindeer moss and 
some other lichens there cover large areas like the grasses In our 
