456 BOTANICAL GAZETTE [june 



bordered pits. Owing to the approximation of the bordered pits 

 and the thickness of the secondary walls, however, these Querleisten 

 are usually more conspicuous when seen in section (fig. 19) than in 

 surface view. 



This scalariform type of tracheary pitting becomes at times 

 considerably modified. Thus the elongated bordering areas of the 

 secondary wall may be replaced by two or more shorter elongated or 

 oval bordering areas (fig. 2). Under these circumstances the pri- 

 mary wall frequently retains its typical scalariform pitting after the 

 secondary wall has lost it; that is to say, each horizontal row of 

 smaller bordering areas is laid down over a single elongated primary 

 pit area (fig. 6). In other cases the elongated bordered pits of 

 the secondary walls may become contracted to form smaller 

 bordered areas which cover only a portion of the surface of the 

 elongated primary pits, and the Querleisten project beyond 

 the outlines of the bordering areas. This process of reduction in 

 the pitting of the secondary wall may even be carried to a point 

 where the primary pit areas have no superimposed bordered pits; 

 or the primary pit areas become less closely approximated, of oval 

 or circular outlines, and separated by relatively wide biconcave 

 thickenings with forking ends (fig. 7). 



Scalariform pitting also grades into types in which there is less 

 unconformity between the primary and secondary walls. The 

 elongated bordered pits become replaced by vertical rows of smaller 

 pits which are staggered so that the pits in one row alternate with 

 those in the next series. These pits are usually superimposed over 

 nearly the whole surface of similar primary pit areas, and the thicker 

 portions of the middle lamella tend to anastomose or form a 

 reticulum, as is shown in fig. 6a. 



Such transitions between scalariform and derived types of 

 tracheary pitting occur in other groups of vascular plants. In 

 certain of the paleozoic and lower mesozoic plants, which had 

 "open" bundles, the metaxylem and secondary wood were com- 

 posed of scalariform tracheids; whereas, in others, the scalariform 

 bordered pits were more or less completely replaced by horizontal 

 or diagonal rows of smaller pits, except in the tracheids of the 

 younger wood of the stele. In the latter types, in passing from the 



