56 THE OAK. 



cally 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 outward 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 strength- 

 ened with spiral thickenings, but are thickened and fur- 

 nished with bordered pits, the shape and characters of 

 which are best seen from the illustrations (Figs. 14-1 G). 

 These larger vessels are not always associated with the 

 radial rows of spiral vessels, but may be scattered be- 

 tween 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, pv). 



The large pitted vessels form groups with parenchyma 

 and wood-cells scattered between, and are confined chief- 

 ly to the inner parts, forming radiating series side by 

 side ; in the outer parts of the bundle are various groups 

 of smaller vessels the groups being rounded, or in ra- 

 dial rows, or curved or oblique rows. 



