54 THE OAK 
cambium is formed throughout the stem, and, as we 
shall see later, throughout the root also. For the 
present it must suffice to notice that the cells of this 
cambium cylinder go on developing into new xylem, 
or phloém, or medullary rays, according to position and 
circumstances ; meanwhile we are only concerned with 
the vascular bundles of the young shoot. 
On the transverse section through the very young 
shoot, provided the preparation is thin and examined 
with a high power of the microscope, the young vascular 
bundles are found to present a definite and symmetrical 
structure, easily distinguished from that of the funda- 
mental: cell-tissue in which they are, so to speak, 
embedded (fig. 12). 
The cells of the medullary rays are seen in one, two, 
or several rows, each cell having the form of a parallelo- 
piped or ordinary brick—the bricks being supposed 
standing on their narrow sides and with the long axes 
directed radially. The walls in contact with the vas- 
cular bundles are thickened, and soon become woody 
and beset with simple pits; the cells contain protoplasm 
and nuclei, and in winter become filled to crowding 
with starch grains. They also contain tannin. 
The young vascular bundles, in section, project into 
the pith—like wedges with a rounded point—giving to 
the latter the five-rayed shape on the transverse section 
already referred to (fig. 9). 
The cells of the pith also have their walls thickened 
and pitted, and also contain protoplasm, nuclei, and 
