128 AGRICULTURAL BOTANY 



nature of the subsoil and the depth of free ground 

 water, but in most annual farm and garden crops 

 the roots may be found in the part of the soil turned 

 by the plow; that is, from six to ten inches in depth. 

 Plants grown in poor soil will have relatively a much 

 more extended root system than those grown in fertile 

 soil. In a dry season a plant will extend its roots much 

 farther in its search for moisture than in a wet season. 



Effect of Trees on Crops. Trees, as we can readily 

 see, must have a wide extension of root surface in order 

 to hold their position and absorb sufficient soil water. 

 As a consequence, their nearness to a crop cannot 

 fail to affect the growth of that crop except in a very 

 wet season, for they absorb from the soil an enormous 

 amount of water and this deprives the plants of what 

 they must have in order to grow; furthermore, their 

 foliage keeps the much-needed sunlight from the 

 maturing grain. 



How Roots Lengthen. Since roots are buried in 

 the soil and the soil is too heavy and dense to be moved 

 by them in large quantities, they cannot grow through- 

 out their entire length. As a consequence, their real 

 life, or growing power, is limited to a short portion 

 just behind the root tip. The root tip is protected in 

 its pushing movement through the soil by a thimble- 

 like covering, called the root cap. This may be seen 

 by the naked eye if the end of a rootlet of a bean grown 

 in water is examined carefully. 



Just back of the center of the root cap the cells in- 

 crease in number by dividing, the separate divisions 

 growing to the size of the parent cell. Each, then, 

 divides into other cells, which in turn grow to normal size, 

 so that the root cap is constantly pushed forward. 



