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 



FlG. 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 bun- 

 dles ; 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 Muller). 



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 ax is- cylinder, nearest the 

 cortex, and the succeeding vessels are formed in ceutri- 



