250 INTRODUCTION TO BOTANY 



these threads absorb food from the algae. The fungus seems 

 to hold the combination in compact form and to enable it to 

 secure a foothold in places where neither the alga nor the 

 fungus could live alone. In the lichen we have therefore a 

 combination of plants an alga and a fungus neither of 

 which alone could live long under the conditions of extreme 

 exposure in which lichens are often found. 



237. Economic significance of lichens. As lichens live in 

 exposed rocky places, they serve to bring about the first stages 

 of soil formation by the decay of old lichen plants and by the 

 breaking up of the surface layers of the rock or other material 





FIG. 195. A small piece of the interior of a lichen, showing the relation of 

 the alga (a) and the fungus (/) 



Magnified 500 diameters. After Bonnier 



upon which they grow. Weathering processes also assist in 

 crumbling the rock, and after a time there is soil enough to 

 permit the growth of larger plants. Several kinds of crusta- 

 ceous lichens are usually the forerunners of other vegetation 

 in rocky regions which will not permit other forms of vegeta- 

 tion to live. The time required to produce enough soil for 

 the growth of other plants depends largely upon the nature 

 of the rock and the climate. It is said that on some lava 

 beds, after almost two hundred years from their formation, 

 crustaceous lichens are still in some places the only plants 

 to be found. Lichens are important as food for herbivorous 

 animals in regions where other kinds of food are scarce or 

 where for a part of the year other vegetation is not available. 



