40 THE OAK 
chief points of difference from the root are that the 
xylem and phloém of these vascular bundles of the stem 
do not alternate on the section, as they did in the root, 
but the phloém of each bundle is on the same radius as 
the xylem; and that there is no pericycle, for branches 
Fic. 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 ands 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 Miller). 
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 
cortex, and the succeeding vessels are formed in centri- 
