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the air — serves to separate them from fungi, which require for food either 
living animals or plants, or else their decomposing remains. In short, given 
a plant whicn is leafless, of some other color than grass-green, aerial, and 
dependent merely for mechanical support on the object to which it is 
attached, and we may be reasonably sure that our plant is a lichen. For 
final decision, in certain cases, a microscopical examination of the internal 
structure is necessary: but to this matter we shall return. 
The extreme simplicity of their needs, and their indifference, for the 
•most part, to cold, heat, drought, and other rigors of climate, make it possi- 
ble for lichens to grow in a great variety of situations, which, but for them, 
would be scarcely occupied by vegetation at all. Wherever a tolerably 
permanent foothold is offered, with a chance of occasional moistening by 
dew or rain and an exposure to pure air and daylight, there one may expect 
to find lichens. They grow on rocks or stones, morter, brick, the bark of 
trees, perennial or evergreen leaves, old wood-work, stumps, and barren 
earth, and occasionally on bones, leather, or bits of crockery exposed to the 
weather. Their distribution over the face of the earth extends from sea- 
shore to the highest mountain-tops, and from the equator to as near the 
poles as man has been able to penetrate. 
Although as a group they are distributed so much more widely than 
most other plants, and appear in general remarkably indifferent to their 
■surroundings, nevertheless there are not a few species of lichens which 
show strong preferences for certain situations, and will thrive only under 
•certain conditions. Thus, there are forms which occur only in northern 
regions or at high elevations, or which produce fruit only in such localities. 
Similarly, temperate and tropical regions have their peculiar forms, and 
likewise sub-Alpine and lower levels. Certain forms of lichens are found only 
on bark, certain others on rocks alone, and others, again, only on the earth. 
There is even with some species a preference for a particular sort of bark, or 
rock or soil. Thus, peaty, clayey, and sandy soils have different species 
growing on them; and calcareous, arenaceous, and granitoid rocks each have 
their own forms, which are rarely found elsewhere. Smooth-barked trees 
harbor lichens different from those on rough-barked trees. Even particular 
species of trees have their own peculiar forms of lichens, which seldom or 
never grow elsewhere. This is true of the cinchonas and certain other trees 
from which medicinal barks are obtained, and pharmacognocists have taken 
advantage of this fact to aid them in distinguishing the different varieties 
of bark imported. 
The amount of moisture to be obtained and the degree of exposure to 
daylight are also important factors in deciding the dwelling place of many 
lichens. It is a fact well known to woodsmen that lichens in this part of the 
world are apt to grow most luxuriantly on the northern or northwestern side 
of tree trunks. This circumstance is often helpful in finding one’s way 
through a forest. The reason why lichens choose one side by preference 
appears to be that, being less exposed to the direct rays of the sun, it 
remains moist somewhat longer after a rain; but exceptions abound. 
