CHAPTER III 



v< 



ROOTS AND THEIR RELATION TO THE WORK OF PLANTS 



16. Structure of roots. We have already discussed the gen- 

 eral nature of roots. Careful examination of a cross section 

 of a young root shows that there is a definitely organized 

 central cylinder, around which is the cortex, both well shown 



in figure 15. The surface layer of the 

 cortex is the epidermis (fig. 16, e). The 

 growing root tip is covered with several 

 layers of dead or dying cells, which make 

 up the rootcap. As the root pushes its way 

 through the soil the rootcap is constantly 

 being worn away on the outside. During 

 the seasons of activity the rootcap is con- 

 tinually being rebuilt by the formation of 

 new layers of cells on its inner surface. 

 In older roots the epidermis has given 

 place to baric, which is composed of sev- 

 eral layers of cells. If a cross section of 

 a very young root of a dicotyledonous 

 plant is examined with a moderate mag- 

 nification, it will be seen that the cortex 

 is made up of rather loosely aggregated 

 cells, while the central cylinder is more 

 closely compacted. In the cylinder are 

 found a definite number of radially arranged fibrovascular 

 bundles. The woody strands of these bundles alternate with 

 strands of what are known as last fibers, shown in figure 44. 



17. The root-hair cell. The general tubular structure of 

 root hairs was very briefly explained in section 8. A root hair, 



20 



FIG. 15. Cross section 



of central portion of 



root of sweet flag 



(Acorus) 



c, cortex; o, outermost 

 layer of cells of central 

 cylinder; v, vessels of 

 various sizes; s, sieve 

 cells. Magnified 70 diam- 

 eters. After De Bary 



