48 THE OAK. 



dies." We have seen that a given strand or bundle may 

 run for part of its course simply side by side with an- 

 other 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 indi- 

 vidual 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 symmetrically disposed between the pith and the 

 cortex, and that the latter (cortex and pith) are in con- 

 nection through the meshes between the interpectinat- 

 ing and concomitant vascular bundles. These radial 

 connections 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 



