462 BOTANICAL GAZETTE [june 



a phenomenon that is concomitant of processes of modification or 

 reduction in tracheary pitting. In the metaxylem of ferns and in 

 the secondary xylem of a number of primitive vascular plants, the 

 primary wall frequently tends to retain its scalariform structure 

 after the scalariform bordered pits in the secondary wall have been 

 replaced by horizontal rows of smaller pits. It may even retain its 

 elongated pit areas and bandlike thickenings after the bordering 

 areas of the secondary wall have been considerably contracted, or 

 have disappeared entirely from certain portions of a facet. On the 

 other hand, when scalariform pitting is replaced by alternate 

 multiseriate pitting, the bandlike thickenings of the middle lamella 

 tend to anastomose and form a reticulum. 



Similar phenomena occur in the metaxylem of many of the higher 

 vascular plants, in the tr&cheids of the secondary xylem of the 

 vesselless dicotyledons Tetracentron, Trochodendron, and Drimys, 

 and in the lateral walls of the vessels of many of the angiosperms. 



This general tendency for the persistence of scalariform pitting 

 in the middle lamella, after it has disappeared more or less com- 

 pletely from the secondary wall, raises an interesting question in 

 regard to the probable significance of the scalariform pitting which 

 occurs so commonly in the middle lamellae of certain of the 

 Abieteae, Taxodieae, Taxaceae, Cupresseae, and Ginkgo, but 

 appears to be entirely absent in the later formed secondary tra- 

 cheids of the Araucarieae. 



It is important to note in this connection that the more primitive 

 vascular plants, which possessed relatively wide zones of primary 

 xylem, were characterized by having numerous closely approxi- 

 mated pits in the radial facets of their relatively large tracheids. 

 In the evolution of the Ginkgoales and Coniferae there appears to 

 have been a more or less pronounced reduction in the amount of 

 primary xylem, in the size of the first formed secondary tracheids 

 of the stele, and in the number of bordered pits in the walls of the 

 tracheary elements. 



The large, thin-walled, heavily pitted tracheids which occur in 

 the' spring wood of the older secondary xylem of mesophytic Conif- 

 erae, resemble the primitive types of tracheids more closely than 

 do the thick-walled, highly specialized elements of the summer wood, 



