The Structures and Processes of Roots 



169 



of the absorption occurs in the root-hair zone, and this is 

 continually moved farther and farther from the base of the 

 stem. In large trees this zone may 

 be many feet from the base of the 

 trunk. 



.^dvantages in spreading roots. 

 The spreading of the root branches 

 in all directions not only enables 

 the roots to come in contact with, 

 more water and mineral salts, but 

 as the woody tissue develops, it 

 helps to anchor the plant more 

 firmly. It is an interesting fact 



that when stems are subjected to 



bending by winds or other agencies, 



the mechanical tissue of the root 



develops to a greater extent than 



ordinarily. 



The principal tissues of roots. 



In the root, the water-conductiag 



tissue and the wood form the 



central axis. Surrounding this is 



a layer of food-conducting tissue. 



In perennial roots there is a cam- 



Growing 



point" 



Region of 

 don^ation 



Root cap 



bium layer between the central fig. 98. Enlarged view of the end 

 axis and the food-conducting tissue. °^ ^ ™°'' showing root cap, growing 



. . region, and root hairs. 



Outside the food-conductmg tissue 



is the cortex, extending to the epidermis. Perennial roots 

 like those of trees soon lose their epidermis ; later the cortex 

 also disappears. The continued thickening of the wood and 

 of the water-conducting and food-conducting tissues results 



