48 



Elementary Principles of Agriculture 



They also take in some of the substances that are dis- 

 solved in the soil moisture. Root-hairs give off carbonic 

 acid gas and possibly other acids, which help to dissolve 

 some substances in the soil. This may be easily demon- 

 strated by allowing roots to grow on a polished marble 

 slab. 



77. The Amount of Root Growth is large. A plant 

 must have a large root surface to absorb enough water 

 to make up for the loss from a large leaf surface. A large 

 leaf surface is, of course, beneficial, because it means so 

 much more surface for absorbing the carbon dioxid and 

 energy from the sun's rays. There must, however, be a 

 balance between the activities of the root surface and 

 the leaf surface. 



78. The Distribution of Roots in the soil varies with 

 the kind and condition of the soil, but, roughly, the 



Fig. 31. When trees are dug up, the large roots are found spreading in the 

 first few feet of soil. These roots had a spread of forty feet. 



