Soil- Makers 35 



upon it when they are removed. The finest hair will 

 leave its mark. 



The amount of acid contained in a delicate little 

 rootlet, or even in a lichen, is, of course, very small — 

 quite insufficient, one would say, for the work it has to 

 do ; and so it is in our hands. That is to say, if we 

 get from the chemist and apply the self-same acid in 

 similar quantities, we cannot do with it what the 

 lichen and other plants do ; we cannot make as much 

 impression upon the rock. For the lichen, though it 

 may look dead, is alive, and the roots are alive ; and 

 living things, however humble, produce wonderful 

 effects, such as dead matter never can. 



But roots also exert a powerful influence upon the 

 rocks in another way. A very common method of 

 breaking up the rocks in use with quarrymen is to 

 drive into them plugs of very dry wood. These plugs 

 are then watered, whereupon they swell with such force 

 as to split even the hardest granite. Roots act in a 

 similar manner, though less violently ; and by swelling 

 in every direction, they gradually widen any cracks 

 into which they have found their way, and actually 

 wedge off large slices from the sides of hills and cliffs. 



In the neighbourhood of Mount Etna people make 

 the roots of the prickly pear work for them in this way, 

 for they want to hasten the breaking up of the lava, in 

 order that they may turn it to account as soon as 

 possible. The lava cracks as it cools, and in every 

 crevice that appears they insert a branch of this cactus, 

 which not only lives, but soon begins to grow, thanks 

 to the warmth, sunshine, and moisture of the genial 

 climate. Its roots cannot, of course, penetrate the 

 lava, but they can and do make their way into every 



