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 i.e. in the tangential plane, and hence, 

 still keeping their mean distance from the epidermis and 

 pith, join with others. 



This being understood, it is also obvious that on the 

 whole the collection of vascular bundles in a young branch 

 form a nearly cylindrical trellis- work or mesh work 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. 



