18 GUIDE TO THE STUDY OF LICHENS. 



the power to burrow into rock of any kind, not even 

 into vegetable tissue ; such a thing is physically im- 

 possible. Nevertheless lichens disintegrate the hardest 

 rock, and it is done approximately in the following 

 manner : Rock-lichens, as well as others, secrete vari- 

 ous acids which have a solvent action upon the less 

 resisting rock-particles. As a result the rock crumbles 

 and the hyphse of the lichen extend between the loos- 

 ened particles. The lichen may thus be almost entirely 

 imbedded in the disintegrated rock-material ; the whole 

 mass being held together and to the unaffected rock 

 the gelatinous hyphse. In time the lichen dies, decays 

 and becomes mingled with the rock-particles, forming 

 a suitable substratum for the higher lichens, such as 

 the large foliose and fruticose forms. In turn these 

 also decay and aid in forming a suitable soil for mosses, 

 ferns and even higher plants ; that is, death and decay 

 gives rise to new life. Throughout nature we find 

 that those living give up their lives that others may 

 live. Lichens are the most altruistic of all living or- 

 ganisms, since they live wholly for the good of others. 

 Nearly all other plants require at least a substratum 

 resulting from the death and decay of others. Many 

 lichens live upon wholly inorganic substrata and exert 

 their entire life-energy for the benefit of other living 

 creatures. 



II. THE ECONOMIC VALUE OF LICHENS. 



We shall here refer to some of the more important 

 uses to which lichens have been put. Other special 

 uses will be referred to in the discussion of the species. 



