The Forms and Structures of Roots 



183 



Structure of roots. In the root, the xylem, made up of water- 

 conducting and wood tissues, forms the central core, or axis. In 

 seedhngs and in a few exceptional plants the root may possess a 

 central pith like the stem. The water-conducting tissue of the 

 root, Hke that of the stem, is composed of tracheae, and tracheids 

 which die at maturity. Surrounding the xylem is the phloem, 

 which is composed of the food-conducting and bast tissue. In 

 perennial roots a cam- 

 bium tissue lies between 

 the central axis and the 

 food-conducting tissue. 

 Outside the phloem there 

 are usually many layers 

 of parenchyma cells, mak- 

 ing up the cortex. The 

 innermost layer of the 

 cortex is often composed 

 of smaller or thick-walled 

 cells and is called the en- 

 dodermis. The outermost 

 layer or layers may like- 

 wise be modified in vari- 

 ous ways, forming cork or 

 coUenchyma. All young 

 roots are bounded out- 

 wardly by an epidermis. 

 In roots like the radish 

 that thicken rapidly this 

 is soon broken. The strips 

 of epidermis remain at- 

 tached to the growing root 

 for some time after it is 

 broken and may be readily ^ t^- . •. • , • 



■^ I'IG. 109. Diagram of a root tip, showing the tissues 

 seen on the radish. and their arrangement. 



Water-con-, 

 ducting 



Growing 

 point 



