104 NOT ALTOGETHER ABOUT PLANTS 



eral chemical condition of the soil in a way which increases 

 its fertility without adding anything to the food-materials 

 which are in it. Thus fertilizers benefit the soil in various 

 ways, and will probably be even more used in the future 

 than they have been in the past. In the future, however, 

 the exact effects which they produce will be better under- 

 stood, and this will lead to a more scientific and economical 

 use of them. 



As in the use of fertilizers, so also in the plowing and 

 harrowing of soil, farmers have increased fertility without 

 realizing just how this increase was caused. It used to be 

 thought that the chief reason for plowing after the plant- 

 ing was done to keep down the weeds. However, it is now 

 known that plowing increases the contact of air with the 

 soil grains, and that to keep the surface of the soil broken 

 up retards greatly the loss of water from it by evaporation. 

 Both of these things are even more important to good crop 

 production than is the keeping down of weeds. 



35. Osmosis. You should be able now to understand 

 something of how substances enter the body of plants. 

 You have been told that water and substances in solution 

 in it enter the roots. You have learned that the principal 

 work of roots is to secure this entrance of water and other 

 substances. You know that the life of the plant depends 

 absolutely upon this entrance. Yet have you any picture 

 in your mind of how this entrance occurs ? It is important 

 to your understanding of plants that you have such a pic- 

 ture, and you should now be able to get it. 



Let your thought go back to the root-hair. There it 

 lies in the soil. Moisture and the soil grains surround it. 

 The great task of its life is to take into itself that mois- 



