POWER OF LIVING STRUCTURES. 99 



used for polishing by cabinet-makers and metal-workers, and 

 still made use of in some parts of the country for scouring 

 pots and pans. Half of their ashes, and in some varieties 

 more than half, consists of flint, so that they are a sort of 

 natural sand-paper, and were at one time largely imported 

 under the name of " Dutch rushes." If examined through 

 a microscope, their cells, regularly arranged lengthwise in 

 ridges, may be seen encrusted with silica ; and if the whole 

 plant be placed in nitric acid, all the soft parts will be eaten 

 away, while the flinty skeleton will remain entire. 



Living structures, whether animal or vegetable, possess 

 extraordinary powers of acting on mineral matter, and a 

 humble lichen will produce more effect on a piece of rock 

 with the sulphuric, oxalic, or nitric acid it contains than an 

 equal amount of acid would do if otherwise applied ; nor 

 would a dead lichen, however acid, be able to do what a 

 living one does. 



What is true in this respect of lichens, is true in their 

 degree of all plants ; and when we consider to what a depth 

 many roots must penetrate, and that wherever they go they 

 effect some chemical change, we can readily understand 

 that, as dust-makers, they are too important to be passed 

 over. 



Moreover, the chemical effects of vegetation are not ail 

 that is to be thought of. The mechanical effects must also 

 be taken into account. 



Few plants are more fragile and delicate-looking than a 

 maidenhair fern, yet one knows that its roots are strong 

 enough to crack the pot if they have not room enough ; 

 weighty kerb-stones have been seen to be forced completely 



