PART II. 



DESCRIPTIVE BOTANY. 



CHAPTER X. 

 ROOTS 



General Characters. Roots are not divided into nodes and 

 internodes and there is no apparent order in the development 

 of branches, as is the case with stems. Root development 

 seems to obey the law of necessity; when a root is needed 

 by a plant it develops and such development may occur on 

 the older as well as on the younger roots. 



Functions. 1 . Roots effectively anchor plants because 

 they consist largely of mechanical tissue, the wood fibers 

 predominating. The horizontal spread of the roots of a 

 plant is usually equal to that of the branches. From the 

 horizontal roots grow numerous vertical roots. The soil 

 surrounding and covering this network of roots adds greatly 

 to their anchoring power. 



2. Roots absorb water. The water-absorbing power of 

 roots is confined to the root-hair zone of the young roots, 

 located just back of the root-cap. Root-hairs are modified 

 epidermal cells. They are small unicellular tubes with a 

 wall of cellulose covered with a thin coat of mucilage. The 

 tube is lined with protoplasm which secretes carbonic acid, a 



