lichens: their economic role. 261 



deeper and deeper as the work of the acid makes a way for them. 

 In many other regions where the rocks have not yet been dis- 

 turbed may be found crustose hchens growing on rock that has 

 not yet fallen to fragments, though so rotten for several inches 

 below the surface that it can easily be ground to fine particles 

 under foot. Thus the work of rock disintegration is aided in its 

 early stages by the lichens and especially by the crustose forms. 

 Here again we do not intend to convey the impression that these 

 lichens are the only agents of rock decay, but merely that they are 

 the first visible organic agency and really play an important part 

 in the process. Atmospheric agencies as rain, winds, changes in 

 temperature, etc., are certainly not to be disregarded. As the dis- 

 integration goes on and the rock at the surface is gradually re- 

 duced to small fragments and soil, the lichens decay and add their 

 quota of humus. Then on this bit of prepared earth, in some 

 crevice or flat surface of rock, the foliose and fruticose lichens 

 and certain mosses begin to appear and carry on the work begun 

 by the crustose species. Then in turn, appear some ferns, herba- 

 ceous seed-plants and finally shrubs and trees, first in the crevices 

 and finally spreading until the whole surface is covered and the 

 lichens are largely replaced by larger vegetation. All this may 

 be most beautifully seen in many places in Minnesota, Wisconsin. 

 Michigan, New England and in other parts of America. 



Let us consider the uses of lichens as food. It is doubtful 

 whether even the wild animals eat lichens to any considerable 

 extent so far south as the pineries of northern United States, for 

 the reason that there is too much of other available plant food. 

 But it would not be surprising if some careful observations in 

 winter would show that the moose, caribou and deer of these 

 regions eat the " reindeer moss," Cladonia rangifcriiia and other 

 large Cladonias to some extent ; for these plants are common and 

 luxuriant. But farther northward the reindeer moss and some 

 other lichens are surely important as food for both man and 

 lower animals. Cladonia rangiferina and two or three closely 

 related species really form the principal food of the Laplander's 

 reindeer and become there as important as some of the grasses of 

 our prairies. This happens for the reason that larger plants do 



