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 phloem, 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 parallele- 

 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 



