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     Many people regard lichens as agents of decay and decomposition . 
     
     but this is not generally true. While a bracket fungus on a beech 
     
     tree penetrates the interior of the trunk and draws its nourishment 
     
     from the trees' reserves, the fungal component of the lichen does 
     
     not need to do this since its foodstuffs are already being supplied 
     
     by the alga within the lichen. Thus lichens usually use a tree 
     
     or stump merely as a place to anchor and do not penetrate it with 
     
     roots or draw moisture or food from it. Tiny threadlike strands 
     
     from the bottom of the lichen serve only to attach it to the substrate. 
     
     Because of the lack of dependence on substrate for food, lichens 
     
     are able to live on extremely inhospitable sites such as mineral 
     
     soil, bare rock and tree bark. Lichens tend to become established 
     
     on sites which are quite long—lasting since their slow growth 
     
     pattern necessitates a substrate which will be around for some time. 
     
    
   
   
    
       
     
    
   
   
    
        
     
          
     
         
     
    
   
   
    
       
     
    
   
   
    
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     CROSS SECTION OF A FOLIOSE LICHEN 
     
    
   
   
    
     Lichens obtain most of their nourishment from minerals and other 
     
     substances dissolved in the moisture of the air. Thus the salt spray 
     
     zone of rocky (i.e., igneous) shorelines is an ideal location because 
     
     it provides a durable substrate as well as plenty of mineral—laden 
     
     moisture. Other groups of lichens have specialized to fit into 
     
     other available habitats. The large yellow patches of lichens some- 
     
     times seen on poplar trees around old farmyards thrive where 
     
     ammonia is present, as it would be near any manure piles. Lichens 
     
     are very sensitive to the chemical conditions in their environments 
     
    
   
   
    
     and many have quite specific requirements and tolerances. One 
     
     group of lichens may live on hardwood trees while another somewhat 
     
     different group will live on the naturally more acidic bark of conifers. 
     
     The chemical sensitivity of lichens makes them useful as indicators 
     
     of air pollution. One of the first changes to occur when air 
     
     pollution causes a slight increase in acidity is the disappearance 
     
     of the acid-intolerant lichen flora from the bark of softwoods. 
     
     This lichen flora on the bark of conifers can no longer tolerate 
     
     the increased levels of acidity there and so these trees become 
     
     lichen barren, while their former lichen complement moves on to 
     
     the hardwood trees whose bark is now sufficiently acidic to provide 
     
     a suitable substrate for lichens from the conifers. Further increases 
     
     in acidity will result in lichens disappearing entirely from the 
     
     area. Aside from sensitivity to acidity levels some lichen species 
     
     are highly sensitive to specific chemicals. This sometimes makes 
     
     them useful as indicator species; the state of health of a certain 
     
     lichen species may provide a rough indication of the levels of a 
     
     particular pollutant in the air. 
     
    
   
   
    
     Lichens have other uses as well. In areas of newly formed 
     
     rock and mineral soil, lichens are often the earliest colonizers 
     
     able to cope with such an environment. Drawing their nourishment 
     
     from the air they gradually introduce a little organic material 
     
     into the area which can later be used by succeeding generations 
     
     of plant colonists. Acids secreted by these lichens help to 
     
     break down rocks and mineral soils and make them more hospitable 
     
     substrate for the higher plants to follow. In the far north the 
     
     lichen known as reindeer moss forms the mainstay of the caribou 
     
     diet. Man has also made use of lichens as a food source: in 
     
     times of food shortage early pioneers and explorers sometimes 
     
     supplemented meagre diets by teas made from lichens, most notably 
     
     the rock tripe. Earlv settlers used lichens for the dying of 
     
    
   
   
    
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