OF THE LICHENS. 133 



admitted fact, that no Lichen is ever submersed, and that 

 they are never developed in mines, caverns, or places 

 deprived of light. In this respect they form a striking 

 contrast to both Algse and Fungi ; the former, as we know, 

 depending for their very existence on the presence of water, 

 while the latter love moisture, and the absence of direct 

 sunlight. 



Lichens, however, prefer the driest and most exposed 

 localities. Within certain limits they bear, with equal 

 indifference, the scorching heat of the midday sun and the 

 icy breath of the north wind. Even when so dried and 

 withered as to crumble in the hand, they will wake to new 

 life when again moistened and placed in such conditions as 

 allow of the development of their cells. This extraordinary 

 persistency, the power of retaining life under circumstances 

 which would be fatal to any other organised being, causes 

 the Lichens to be veritable pioneers in the vegetable king- 

 dom. Taking possession of the bare face of a cliff, where 

 not even a moss could find footing, they seize the passing 

 dew or raindrop, introduce it into the shallow fissures of 

 the rock on which they are clustered, and thus gradually 

 prepare a foothold for the higher plants. Nor is their 

 usefulness limited to their living state ; their very death is 

 made subservient to the great purposes of nature, by filling 

 the cavity on which they grew with the humus formed out 

 of their own corruption ; doubtless a trifling amount, but 

 sufficient to support the tiny moss which succeeds them, 

 and which in its turn lives and dies only to supply 

 nutriment for some more highly organised plant. Perhaps 

 the most remarkable evidence of the powers of vitality 

 belonging to this class is shown in the fact, that no sooner 

 do the streams of lava begin to cool after a volcanic 

 eruption, than a Lichen (known as Stereocaulon Vesu- 

 vianum) spreads its hard solid thallus over the glowing- 

 surface. 



Most of the Lichens prefer to grow on either stone or 

 wood ; very few flourish an the naked soil ; a small propor- 



