52 THE OAK 



CHAPTER V 



THE SEEDLING AND YOUNG PLANT (continued) 



STRUCTURE OF THE VASCULAR TISSUES, &c. 



BEFORE plunging into the intricacies of the vascular 

 bundles it will be well to obtain some idea of the 

 general plan of structure which they present on trans- 

 verse section (fig. 9). As already seen, each of the bundles 

 of the ring consists of a xylem portion on the side next 

 the centre of the stem, and a phloem portion on the side 

 next the periphery, and these portions are separated 

 by the cambium layer. The tissue in the centre of the 

 stem, and surrounded by the ring of bundles, is called 

 the pith ; the tissue outside the ring, and between it 

 and the epidermis, is called the cortex ; and the tissue 

 left between the bundles is termed the primary medul- 

 lary rays (fig. 9). 



It will, of course, be remembered that the term 

 ' ring,' as used above, always expresses the fact that a 

 cylinder is here viewed in section. Now, the cambium 

 of the individual bundles soon unites across the primary 

 medullary rays, and thus a complete hollow cylinder of 



