ANATOMY OF THE OLDER PORTIONS OF THE ROOT 149 



equal in number to those on the outside. The new cells next to 

 the xylem become modified into xylem, at first filling in between 

 the strands of the primary xylem, and the cells formed next to 

 the phloem are changed into phloem. In this way the vascular 



FIG. 132. Diagrams showing how the xylem and phloem of roots are 

 increased. A, cross section of a root showing xylem (x) and phloem (p) 

 before the cambium is formed. B, cross section of root showing xylem (x) 

 and phloem (p) after the cambium ring (c) is formed. C, cross section show- 

 ing the xylem (z) and phloem (p) shown in A and B and the new xylem (a) 

 and new phloem (6) which have been formed by the cambium (c). Adapted 

 from J. M. Coulter. 



cylinder is increased in diameter as shown at C. In roots which 

 live many years, like those of trees, the layers of xylem formed 

 each year form annual rings like those occurring in woody stems; 

 and the outer layers of the phloem, with the cortex and other 

 tissues outside of the phloem, constitute a bark like that of 

 woody stems. In fact, it is only by the presence of their early 

 root anatomy that sections of such roots can be told from sec- 

 tions of stems. In some fleshy roots, as Beets illustrate, a num- 

 ber of cambium rings form outside of the first one, and the 

 growth resulting from each appears as a ring when a cross-section 

 of the root is made. 



