128 RHIZOME OF FTERIS. [CH. IX 



bedded or packed as it were in a small quantity of 

 parenchyma. There are a few minute spiral vessels (not 

 shown in the figs. 54, 55), but the great mass of xylem 

 vessels are of large diameter and are known as scalari- 

 form vessels. 



The scalariform, i.e. ladder-like character of these 

 vessels comes out clearly in longitudinal section. The 

 horizontal markings which represent the rungs of the 

 ladder are thickenings of the walls. 



