250 INTRODUCTION TO BOTANY 
these threads absorb food from the alge. 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 
Fie. 195. A small piece of the interior of a lichen, showing the relation of 
the alga (a) and the fungus (/f) 
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. 
