66 



OAK. 



[CH. V 



The development depends upon the activity of the 

 cambium, which in the seedling oak has the same form as 



FIG. 28. 



TRANSVERSE SECTION OF A FIVE-YEAR-OLD OAK-BRANCH in which the isolated 

 bundles are replaced by concentric shells of wood. 

 p, pith. rn.r, primary medullary ray. 



Xj to x 5 , shells of xylem formed during successive years. 

 The secondary medullary rays are not shown. 



in the sunflower, namely, a cylindrical shell looking like a 

 ring in transverse section. 



As in the sunflower so here the cambium manufactures 

 cells on its inner side which become xylem, and cells on 

 its periphery which become phloem. It therefore follows 

 that the cambium manufactures a cylindrical shell of 

 xylem on one side and a cylindrical shell of phloem on 

 the other. 



As already mentioned the pith can be seen as a small 

 speck (fig. 29) in the centre of the section of an oak-tree, 



