iqiq] bailey— bars OF SANIO 459 



specialized types the primary wall (the first formed portion of the 

 vessel member) tends to retain its primitive elongated type of 

 primary pit areas after the scalariform bordered pits have become 

 locally constricted (fig. 4), or divided into horizontal rows of smaller 

 pits (fig. 9). Similarly, in those wa;lls where there is a tendency to 

 eliminate the bordered pits, the elongated primary pit areas persist 

 after the bordered pits have partially or completely disappeared 

 (figs. I, 8, 9). 



In the highly specialized vessels of the Anonaceae and Laura- 

 ceae (fig. 10) there are numerous circular or oval bordered pits in 

 the lateral walls of the vessels. Usually they appear to be laid 

 down over similar circular primary pit areas. In other words, in 

 the most highly specialized types of vessels, in which the bordered 

 pits are not arranged in horizontal rows, even the elongated primary 

 pit areas are more or less completely obhterated, and replaced by 

 attenuated areas with circular outlines. Vestiges of Querleisten, 

 however, are sometimes present near the upper and lower margins 

 of the bordering areas. 



In the dicotyledons, with increasing specialization of the 

 vessels, there is a corresponding reduction in the pitting of the 

 remaining tracheary elements. Thus tj^ical tracheids are replaced 

 by fiber tracheids, which are in turn replaced by libriform fibers. 

 The fiber tracheids of certain dicotyledons have elongated or oval 

 primary pit areas that are separated by wide dark colored bands. 

 Furthermore, it is not uncommon to find that many of the attenu- 

 ated areas of the middle lamella have no superimposed bordered 

 pits (fig. 22). 



Unconformity of the type that occurs in the tracheids of various 

 primitive vascular plants, and in the vessels of certain dicotyledons, 

 has been observed in the wood of Ginkgo, and certain of the Abieteae 

 and Taxodieae. Fig. 23 illustrates a type of tracheary pitting that 

 is of frequent occurrence in the older secondary wood of vigorous 

 mature specimens of Ginkgo and Taxodium. It is most character- 

 istically developed in large thin-walled tracheids of the so-called 

 spring wood. The numerous, uniformly narrow, elongated primary 

 pit areas and thin, straight, narrow Querleisten are typically 

 scalariform in structure. Each primary pit area has superimposed 



