LICHENS. 169 



tend to show that the species are interesting to the geologist as 

 well as to the botanist. The writer states, " almost all lichens are 

 more or less intimately united to the bodies on which they grow," 

 the surface of the latter is frequently pierced or broken up by 

 the tissues of the lichen ; nay, the hardest calcareous rock, the 

 smoothest quartz, is corroded and disintegrated ; and deeply sunk 

 in their substance we find the fructification of several species of 

 Verrucaria and Lecidea. This phenomenon has hitherto been 

 unexplained; it probably depends on some chemical action 

 exerted on the rock by the lichen." In reference to the for- 

 mation of soil, the same writer remarks, that " lichens form a soil 

 fitted for the germination and growth of higher plants a saxi- 

 colous, crustaceous species growing on the bare quartz summits 

 of the highest mountains ; or, he adds, we may suppose its 

 habitat to be the bare lava of a volcanic district, or the equally 

 sterile surface of a newly upraised coral island. The delicate 

 spores of such a species have been wafted thither by a breeze or 

 washed to its surface by a shower. They germinate, and develop 

 a thallus, which becomes adherent to the rocky surface by a pro- 

 cess of disintegration. Erom the atmosphere chiefly, and from 

 the rock perhaps to a slight extent, the plant derives nourish- 

 ment, grows, and in the course of time dies, thereby adding to 

 this stratum of mineral soil, which it has produced, a thicker 

 layer of vegetable soil, This soil is now suited for fructiculose 

 or foliaceous lichens ; these in their turn decay, and contribute 

 to the increase of the vegetable soil, which is next taken posses- 

 sion of by mosses and ferns, and gradually, by various phanero- 

 gamic plants? shrubs, and trees Linnaeus denomi- 

 nated the mosses, servi handmaids of nature. We think the 

 reader will agree with us in considering that the lichens have a 

 superior claim to the appellation." With regard to the habitat 

 of the genus Lecidea,, it may be remarked that it is found at the 

 greatest elevation hitherto reached by man ; it occurs far above 

 the line of perpetual snow on the Alps, and is the last type of 

 vegetation met with on the Andes and Himalayas, and on the 

 deserts of Nova Zembla. 



(7 and 8) Opegrapha and Graphis are two genera of minute lichens 

 that have acquired their respective names from the resemblance 

 of the species to written characters. 0. vulgata and G. scripta 

 w 



