56 THE OAK 
of longitudinal sections also show that these vessels at 
first simply consist of longitudinal rows of very narrow, 
vertically placed, cylindrical cells, standing end to end; 
it is because the adjacent ends become resorbed and 
disappear that the rows of cells at length form long 
continuous tubes—vessels, or tracheee. 
Turning once more to the transverse section, as 
the eye follows the bundle radially outwards the 
lumina of the vessels in the radial rows are found to 
become wider and wider, until we meet with vessels with 
diameters many times greater than that of the pith- 
cells. The walls of these wider vessels, however, are 
not strengthened with spiral thickenings, but are 
thickened and furnished with bordered pits, the shape 
and characters of which are best seen from the illustra- 
tions (figs. 14-16). These larger vessels are not always 
associated with the radial rows of spiral vessels, but 
may be scattered between them. 
The vessels intermediate between the spiral and the 
pitted ones are thickened sometimes with reticulations. 
All these larger vessels have septa inclined towards the 
medullary rays, and perforated with several long, oval, 
parallel, horizontal holes: hence the segments are easily 
macerated and distinguished, and their lengths are found 
to be variable (fig. 16, p.v). 
The large pitted vessels form groups with paren- 
chyma and wood-cells scattered between, and are con- 
fined chiefly to the inner parts, forming radiating series 
side by side; in the outer parts of the bundle are 
