48 THE OAK 
for part of its course simply side by side with another 
and separate from it; at other parts of the course the 
bundles may be united with others. In the case of the 
oak it will be clearly borne in mind that the individual 
or separate bundles of the leaf-trace pass into the stem 
at the node of insertion of the given leaf, and then run 
down side by side at a practically constant distance from 
the surface of the epidermis on the one hand, and the 
longitudinal axis of the pith on the other. At different 
levels below, at or very near nodes, these bundles turn 
aside laterally—z.e. in the tangential plane, and hence, 
still keeping their mean distance from the epidermis and 
pith, join with others. 
This being understood, itis also obvious that on the 
whole the collection of vascular bundles in a young branch 
form a nearly cylindrical trellis-work or meshwork sym- 
metrically disposed between the pith and the cortex, 
and that the latter (cortex and pith) are in connection 
through the meshes between the interpectinating and 
concomitant vascular bundles. ‘These radial connec- 
tions of the pith and cortex are the primary medullary 
rays. 
It will now be clear why we observe on transverse 
sections of the young stem taken across an internode 
the arrangement shown in fig. 9. The vascular bundles 
are grouped in a ring round the pith, separating it off 
from the cortex and its covering the epidermis, and with 
those primary medullary rays which happen to have 
been cut running between the bundles. 
