io6 



ELEMENTARY SCIENCE 



der-like spores, which reproduce the plant. Toadstools, 

 mushrooms, and puffballs are examples of such structures 

 (see Fig. 47). 



The effects which such plants produce upon the fer- 

 tility of soil are not well understood, but it is known that 

 their threads penetrate everywhere in soil that contains 

 organic matter, and they certainly produce many chemical 

 changes in such soil. Some of them form felt-like coat- 



FIG. 47. Puffballs. 



ings on the tips of the roots of certain plants, and seem 

 to be of benefit to such plants (see Fig. 48). 



Enough has been said to prove to you that soil is a very 

 complex thing. Except where it is quite bare of plants, 

 like bare sand, it contains more or less of organic material, 

 and this organic material furnishes food for hosts of micro- 

 organisms. These micro-organisms, along with the physical 

 and chemical nature of the soil, and its relations to water, 

 air, and heat are all factors in determining fertility or 

 infertility. 



