458 BOTANICAL GAZETTE [june 



bordered pits. These transverse strips are deeply stained portions 

 of the middle lamella that stand out in sharp contrast to the thin 

 pit membranes. They are, in fact, very narrow, bandlike thicken- 

 ings {Querleisten) which separate the elongated primary pit areas. 

 In the case of vessels withmultiseriate pitting, these transverse 

 ridges or Querleisten tend to occur between the elongated primary 

 pit areas that underlie the horizontal rows of bordered pits. In 

 other words, a single elongated bordered pit may be laid down over 

 the whole surface of the elongated primary pit area, or one or more 

 smaller bordered pits may be laid down over portions of its surface. 

 Under the latter circumstances, the outlines (Umrissen) of the 

 primary pit areas become more conspicuous (figs. 4, 9, ii). In 

 certain cases these Querleisten become more or less completely 

 divided into shorter, rodlike thickenings, which lie between the 

 upper and lower margins of contiguous bordered pits of the vertical 

 series. 



It was shown by Strasburger (15) that bordered pits are not 

 laid down over all the primary pit areas in the tracheids of Pinus 

 and Larix. In the Magnoliineae and other dicotyledons the lateral 

 primary walls of the vessels frequently have elongated attenuated 

 areas, which have no bordered pits superimposed over them, 

 or only a comparatively limited part of their surface so covered 

 (figs. I, 8, 12). 



In the Magnoliineae, Trochodendrineae, and other groups of 

 dicotyledons there is much evidence to indicate that scalariform 

 pitting is a relatively primitive feature in the structure of vessels. 

 That is to say, those vessel segments which most closely resemble 

 tracheids in general form and structure tend to have scalariform 

 or scalariform and opposite multiseriate pitting; whereas the larger 

 and more specialized conducting passageways are characterized 

 by having alternate multiseriate pitting in their lateral walls 

 (fig. 10). 



In the evolution of larger and more specialized vessels the modi- 

 fication of the primary walls does not appear, in many cases, to 

 have kept pace with that of the secondary walls. Thus in primitive 

 types of vessel segments the elongated bordered pits are exactly 

 superimposed over similar elongated primary pit areas, but in more 



