36 Soil-Makers 



crack and cranny within reach, and as they grow and 

 swell they break up the rock into fragments. 



As to what the plant lives upon in the absence of 

 -soil, it must be remembered that often a very little 

 mineral food is enough for a plant, if only it is able to 

 make the most of what there is and has plenty of 

 water ; then we must remember, too, that lava is 

 especially rich in the materials required by plants, and 

 that water flowing over it, or draining through it, 

 would certainly dissolve some of these materials and 

 bring them within reach of the roots, which would 

 obtain them in this way quite as well as from soil ; 

 and then, finally, the whole cactus family, like the 

 lichens, are furnished with large quantities of acid, 

 by which they take up an altogether unusual amount 

 of mineral matter, and they are apparently able to 

 take it up, as the lichens do, from the solid sub- 

 stances, at least, to some extent, for in their native 

 land, America, they are to be seen growing out of 

 the rocks. 



Though we may not have had the opportunity of 

 noticing how roots split the rocks, we are all probably 

 familiar with the power which they exert on a smaller 

 scale and upon humbler material. Who that has 

 • gardened,' even to the extent of growing a few ferns, 

 but has some time or other known a pot cracked by 

 the efforts of the roots to find room when they have 

 outgrown their domicile ? 



But we have now to consider other means by which 

 nature prepares the soil. Hitherto we have confined 

 our attention to what is done with the rocks on the 

 spot, the soil being left where it is made ; but this pro- 

 ceeding is attended by certain disadvantages : the soil 



