CHAPTER XII. 



LICHENS AND THE ROCK FLORA. 



Crottles or lichens are a very exceptional group. 

 They consist of two distinct and different plants, an 

 Alga and a Fungus ; which, by combining forces, are 

 able to persist and to flourish in places where neither 

 could exist separately. The Algae are unable to do 

 without a plentiful supply of water. Most of them 

 are permanently submerged, while the rest occur 

 either in places which are kept almost perpetually 

 moist by trickles of water, or amongst damp moss. 

 The Fungi are, as a class, perhaps more delicate than 

 any other ; a single night's frost will utterly destroy 

 all the Agarics which are in fruit, and very dry 

 conditions seem almost as fatal as frost. Yet the 

 lichens, formed of an alga and a fungus, are able to 

 endure the worst weather, and to stand exposure to 

 every sort of climate without the least injury. A 

 lichen-covered boulder on a Highland mountain will 

 suffer in one year the most extraordinary vicissitudes. 

 During the winter severe frost continues for weeks 

 together. Sometimes rain falls almost every hour for 

 a month or more, and during that time the lichens 

 are submerged. In summer there may be three or 

 four weeks of dry weather in which scarcely any 

 moisture, except that from dew, reaches the lichens, and 

 on sunny days the boulders often get so heated by 



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