68 APPLIED BIOLOGY 



these functions ? In order to answer such a question, even 

 in part, it is necessary that we study first the structure of 

 the organs of a plant. For this reason we will examine the 

 roots, stem, buds, leaves, flowers, and fruit of the bean 

 plant. 



The Bean Roots 



67. General Structur of Roots. (L) Carefully dig up a young 

 bean plant, and note how firmly the roots anchor it in the soil, 

 and how the particles of soil cling to the small rootlets. Wash by 

 dipping into water. Note that there is no definite boundary between 

 roots and stem. The small rootlets appear to be attached to the 

 blunt lower end of the stem and also to a central root which seems 

 to be the downward continuation of the stem. 



Some of the stem is below ground. That all the parts of 

 the plant below the surface of the soil do not belong to the 

 roots, as is popularly believed, is evident if we compare the 

 roots of a bean plant grown from a seed planted four inches 

 deep with those on a plant from a seed placed one inch deep 

 in the soil. In both plants the roots are grouped at about 

 the same place, and the deeper planted one has more than 

 three inches of stem below the surface of the soil, but at 

 first showing no roots. After growing six or eight weeks, 

 other roots usually start from the stem nearer the soil sur- 

 face. It is well known that stems of many kinds of plants will 

 form roots if they are kept in contact with moist soil. 



Usually the main root is called primary root, and the 

 branches secondary roots. 



On roots of some bean plants there may be seen small 

 spherical bodies, some of them perhaps as large as one eighth 

 of an inch in diameter (Fig. 25). These are root-tubercles. 

 In the later lessons on bacteria (a kind of microscopic plants), 

 these tubercles will be referred to as caused by bacteria and 

 useful as makers of a peculiar kind of plant food supple- 

 mentary to that furnished by manures or fertilizers in the 

 soil. 



