254 ECONOMIC VALUE OF LICHENS 



lichen was produced artificially by bringing together a suitable 

 fungus and alga. It may also be stated that small lichen-like 

 bodies have been developed on glass plates from fungi alone by 

 feeding them suitable nutrient solutions. This fact that the 

 fungus assumes in its growth the form of a lichen thallus shows 

 that the fungus has become changed in its nature through associa- 

 tion with the algae. 



(c) Economic Value. — Lichens are of considerable economic 

 importance in the world. Many genera live upon the rocks of 

 which they appear to form an integral part. They are usually 

 the first vegetation to appear in such places. The hyphae pene- 

 trate the irregularities of the rock and even efTect disintegration 

 of its substance. Because of this and also since they serve to 

 collect the dust and other material that is washed down by rains, 

 they are very important factors in preparing a soil suitable for 

 other plants. In many sections they form the characteristic 

 vegetation of the country, as in certain alpine and desert regions 

 and in the northern barrens, where they afford rich pasturage to 

 reindeer and caribou. Their abundant growth in mountainous 

 districts and subsequent distribution by winds and rains accounts 

 for the showers of manna in biblical history. Certain species are 

 still used in northern regions to lengthen out a meager food sup- 

 ply. Litmus, employed in acid testing, and various pigments are 

 derived from lichens, as were also the famous purple and blue 

 dyes of the East. 



