92 THE WORLD'S LUMBER ROOM. 



dry, and no soil seem to come amiss to them, for they will 

 grow loose on the surface of the sand in Peru, and attach 

 themselves to the dry bones of mules which have died by the 

 wayside. They are invariably the first plants to make their 

 appearance, whether upon lava or upon the rocks of islands 

 newly raised above the sea, and no rock is too hard for them. 

 The multitudinous spores by which they are propagated are 

 for ever floating in the air, and being furnished with a 

 gummy fluid are able to attach themselves to the barest, 

 smoothest surface ; and when once they have gained a foot- 

 ing, they are simply irresistible. 



They flourish on granite, slate, lava, and in Berkshire 

 (Mass.) even the white quartz hills are covered on their 

 moister slopes with large patches of a leathery lichen, which 

 adheres so firmly that it can hardly be detached from the 

 stone. 



The first lichens to appear are, as has been said, mere 

 stains ; but, by the growth and decay of successive genera- 

 tions of these, a thin film of soil is formed, upon which 

 larger kinds take root in their order, and at last one may 

 see rocks, or old tombstones, covered with a crust of lichen 

 an inch or more thick. On this crust mosses begin to grow, 

 and they help on the process of decay by keeping the surface 

 moist and sending their roots down into the stone ; then 

 insects collect and feed, die and decay, and thus the mineral 

 matter of the rock is not only reduced to powder but mixed 

 with organic remains, without which it would be quite unable 

 to support the higher orders of plants. 



The mosses, as they grow thicker and thicker, keep the 

 air from the rock, and thus to a certain extent protect it ; on 



