28 The Great World's Farm 



ways), but they could not have done what the hchen does; 

 and but for the hchen, they could never have grown here 

 at all. 



Lichens, as we have said, are able to eat into the rock, 

 as oxygen eats into iron, and by similar means, for both 

 are strongly acid; and on removing lichens from a stone 

 one sees indentations similar to those left on a piece of 

 iron by the removal of rust. But what is true of lichens 

 is true in a degree of all plants. The roots of all plants, 

 that is to say, are acid, though in a less degree, especially 

 the young, fine, hair-like roots; and if these find their way 

 through the thin soil to the rock beneath, they eat into it, 

 leaving a distinct impression of themselves upon it when 

 they are removed. The finest hair will leave its mark. 



But roots also exert a powerful influence upon the 

 rocks in another way. A very common method of break- 

 ing 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 spht 

 even the hardest granite. Roots act in a similar manner, 

 though less violently; and by swelhng 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 neighborhood 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 



