258 



ELEMENTARY STUDIES IN BOTANY 



wood) that alternate with the original strands of phloem. 

 The vascular bundles of the root connect with those of the 

 stem, and these in turn with those of the leaves, so that 

 throughout the whole plant there is a continuous vascular 

 system. 



The origin of the branches of roots is very different from 

 that of stems. In a stem the branch begins at the outer 

 part of the cortex, but in the root it begins at the surface of 



FIG. 224. Diagram to show how roots increase in diameter : A represents cross-sec- 

 tion of a young root in which four phluem strands (p) alternate with four xylem 

 strands (x) ; B represents an older root in which there is a continuous zone of 

 cambium (c) that is forming on the outside new phloem (np) in contact with the old 

 (p), and on the inside new xylem (nx) alternating with the old (x). 



the vascular cylinder and breaks through the cortex (Fig. 

 225). If the cortex of a root be stripped off, the branches 

 will be found attached to the central axis, and the perfora- 

 tions made by the branches through the cortex can be seen. 

 142. Growth in length. The elongating region of the 

 root is much more restricted than that of the stem. It was 

 stated ( 135, p. 244) that the elongating region of a stem 

 may extend ten or twenty inches from the tip, or even more ; 

 but the elongating region of a root is hardly ever more than 

 two-fifths of an inch, and often not more than half of that/ 

 The region of elongation and of greatest elongation should 



