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 Algce 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 on the naked soil ; a small propor- 
