PITTED VESSELS. 



77 



CH. V] 



Longitudinal (Radial) Section. 



The large xylem vessels are again the most striking 

 feature. They appear in longitudinal section as empty 

 spaces with here and there a remnant of an oblique 

 transverse wall. The markings on the walls can be seen 

 well where fragments of membrane come into the section 

 as shown in fig. 36. 



Fm. 36. 

 PAET OF A DOTTED XYLEM-VESSEL FROM THE OAK. 



These markings, from which the vessels take the names 

 of " dotted " or " pitted," are like so many screw-heads, a 

 disc traversed by a transverse line or elongated mark which 

 represents the groove for the screw-driver. The structure 

 of these pits, which are known as bordered pits, can be 

 explained by an imaginary model. Imagine a pair of 

 watch-glasses each pieced by a narrow slit, and imagine 

 them united face to face with a delicate circular piece of 



