84 TRACHEARY ELEMENTS 



Pseudo-Acacia) occurs after the cells have reached their full size 

 and have developed secondary walls over all portions of the ele- 

 ment. Perforation in this species therefore involves the dissolu- 

 tion of a portion of hoth the secondary as well as the primary 

 layers from the central region of the end-walls. Esau (1936), 

 however, in her study of vessel development in the primary 

 xylem of celery, found that the secondary thickening', in the form 

 of spiral bands, is restricted to the lateral walls and to a thin 

 peripheral region of the transverse end-walls. The perforation in 

 this case involves the breakdown of a distinct central region of 

 the end wall which is interpreted as primary in nature. Tliis area 

 in section view appears as a lenticular thickening and is "similar 

 to the torus tliickening in bordered pits." In a later publication. 

 Esau and Hewitt (1940) investigated the nature of the end-walls 

 and the development of perforations in the vessel elements of 

 Cucurhita pepo, Zea mays, Nicotiana tahaciim, Daucus carota and 

 Beta vulgaris. The end-walls of Beta, Daucus and Nicotiana 

 agree with those in celery in possessing a conspicuous lens-shaped 

 thickening which breaks down to form the simple perforation 

 during the final stages of vessel development, after secondary 

 walls have been formed. Perforation of tlie vessel elements in 

 Cucurhita occurs at a similar period in ontogeny but in this form 

 the portion of the end-wall to be dissolved is not lens-shaped in 

 sectional view. As a' result of careful microchemical and optical 

 tests of the end-walls. Esau and Hewitt conclude that "two 

 superimposed vessel elements are separated from each other by 

 two cellulose layers — the two pi-iniary walls — cemented together 

 by isotropic intercellular substance." 



(h) The nature and origin of "fibrous" thicl-eni)igs in pri- 

 mary xylem tracheae. The secondary wall of the tracheary ele- 

 ments in ])i"imai'y xylem is deposited ujion the delicate ju-imary 

 wall in a number of well-defined patterns. In the proto.rylem or 

 first-formed jiortion of the ]U'imary xylem, the secondary wall ap- 

 pears in the form of separate rings (annular elements), one or 

 more spiral bands (spiral elements) or as transverse, intercon- 

 nected bars (scalariform elements). These distinctive types of 

 tracheary elements usually originate successively in provascular 

 tissue ("procambium") in the order named above, although the 



