136 ETON NATURE-STUDY 



Even in our own islands there are several districts of which the 

 surface rocks consist of nothing but vast sheets of cooled lava, 

 erupted at a comparatively recent epoch. No one who ascends an 

 active volcano such as Vesuvius will fail to be struck by the 

 waste of black rock with which the mountain slopes are clothed. 

 Nothing, it might be thought, could grow on such a barren-looking 

 surface. Yet sooner or later the black basalt will be found to 

 give way to a dull grey rock which, if more closely examined, will 

 be seen to be stained a variety of colours. This appearance is 

 produced by an encrusting lichen which marks, if we omit bacteria, 

 the first vanguard of the vegetable world in attacking the newly 

 formed rock. These lichens are next followed by mosses, which 

 still further decompose the rock, turning lifeless inorganic substance 

 into living tissue. Soon sufficient amount of soil is accumulated 

 to give other plants, ferns, and grasses a hold ; and seeds are 

 brought by the wind and the other agencies alluded to in Chapter I, 

 Part I, until little by little the barren waste of lava becomes a fertile 

 and smiling landscape. 



It is obvious, therefore, that lichens are among the hardiest of 

 living plants. They can exist and flourish clinging to bare rock, 

 such as on a mountain top is exposed to the most trying changes 

 of weather. At one time scorched by the sun, at another drenched 

 by the rain, and dried again by furious winds, these lowly plants, 

 nevertheless, flourish in places where no other vegetation could even 

 obtain a foothold. 



What then can be the nature of the lichen ? 



With a strong hand-lens the student will probably be able to see 

 from the cut edge that the lichen is a combination. Two distinct 

 plants, a fungus and an alga are living together, and form a 

 complex \vhich can live in places where it would be quite impossible 



