40 



THE OAK. 



chief points of difference from the root are that the 

 xylem and phloem of these vascular bundles of the stem 

 do not alternate on the section, as they did in the root, 

 but the phloem of each bundle is on the same radius as 

 the xylem ; and that there is no pericycle, for branches 



FIG. 9. Transverse sections through very young twigs of oak, showing 

 the vascular bundles of the stem (P and X), arranged in a ring round 

 the pith, and joined by the cambium ring the fine line passing 

 through the bundles ; M and , the vascular bundles passing down 

 from the leaves M the median bundles and s the lateral bundles. 

 The external outline is the epidermis ; the letters P P stand in the 

 primary cortex ; the letters X X stand in the pith ; the primary 

 medullary rays separate the bundles. (After Mliller.) 



are not developed endogenously as rootlets are. Then 

 there are some important differences in the mode of 

 origin of these vascular bundles in space. We saw that 

 in the root the first-formed spiral vessels are developed 

 at the outer parts of the axis-cylinder, nearest the cor- 

 tex, and the succeeding vessels are formed in centripetal 

 order from these points. In the young stem the exact 



