286 



ELEMENTS OF GENERAL SCIENCE 



lichens and mosses, grow upon the surfaces of rock, and the 

 organic products (acids) formed by them assist in disintegrat- 

 ing the rock. When once a little broken rock and organic 

 soil material has been formed, other plants and animals may 

 then grow and may in their turn make further contributions 

 to soil-making. Unless this soil is washed away, it may con- 

 rtantly increase in its depth and productivity. 



FIG. 141. Action of plants upon rocks 



The roots of plants enter crevices and break the rock. Here there is not enough 

 soil to cover the rock, and the pine trees are rooted in the sandstone 



286. The soil and its significance. It is customary to speak 

 of the soil as including the top layer of earth material. The 

 term is sometimes used to apply to whatever earth materials 

 there are upon the solid rock. In agriculture and garden- 

 ing the term applies to the more nutrient layer in which 

 plants grow. 



The soil is the natural growing place of most plants and 

 many animals. The roots of plants are usually embedded in 



