MINERAL FOOD OF PLANTS. 93 



the other hand, they also keep the surface moist, and as 

 moist surfaces absorb more carbonic acid than dry ones, 

 this also helps forward the decay to some extent, even though 

 the other enemy frost be kept out. 



Now, how do the lichens manage to wear away the rocks, 

 since, though low down in the scale, they are certainly 

 plants, and plants can live only upon liquid food ; yet these 

 eat their way not only into bricks which have been baked, 

 but into rocks which have been vitrified, converted, that is, 

 into a sort of glass by the heat of the subterranean fires ? 



On removing a lichen from the rock, we see that it has 

 made an impression more or less deep, and that the stone 

 has lost so much of its substance ; and further, on burning 

 the lichen, we find that it contains from ten to twenty per 

 cent, of solid ash, which will not burn away, being, in fact, 

 mineral matter abstracted from the rock. 



How then has the lichen managed to feed upon stone ? 

 All plants have the power of forming within themselves 

 acids of one sort or another,* and as lichens do this to a larger 

 extent than most others, they are able to dissolve the very 

 hardest rocks. Many of them, indeed, so abound in oxalic 

 acid that oxalate of lime makes up half their weight, and 

 many of the old Greek marbles are thickly encrusted with 

 this substance from the growth and decay of lichens on their 

 surface. Oxalate of lime is simply a combination of oxalic 

 acid and lime ; the acid having dissolved the lime from the 

 rock or marble, and so brought it into a condition in which 

 the lichen could absorb it. 



But though lichens are the first soil-makers and take up 



* See Chap. XI. -Vegetable Scavengers. 



