DEVELOPMENT OF PLANTS 241 



bodies have been developed on glass plates from fungi alone by 

 feeding them suitable nutrient solutions. 



(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 effect disintegration 

 of its substance. They also serve to collect the dust and other 

 material that is washed down by rains and so prepare a soil for 

 other plants. In many sections they form the characteristic vege- 

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

 in the northern barrens, where they afford rich pasturage to rein- 

 deer and caribou. Their abundant growth in mountainous dis- 

 tricts 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 

 dves of the East. 



17 



