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 tracheae. 



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 



