52 THE OAK 
CHAPTER V 
THE SEEDLING AND YOUNG PLANT (continued) 
STRUCTURE OF THE VASCULAR TISSUES, &. 
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 phloém 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 
