492 



CYCADOPHYTA 



[CH. 



zone is the secondary xylem the inner edge of which forms bays, 

 and this is composed of alternating zones of vertical and horizontal 

 tracheids (fig. 580, y x 5 ; fig. 581) with bordered, scalariform, 

 pits on their walls traversed by medullary rays generally biseriate 

 and from 4 to 30 cells deep. The disposition of the tracheids is 

 such as to render transverse and radial longitudinal sections 



pm. 



FIG. 580. Colymbetes Edwardsi. Transverse section showing ten alternating 

 zones of wood outside the perimedullary zone, pm; y lt y z , etc., vertically 

 running xylem series; x lf x 2 , etc., horizontally running xylem series; p, pith; 

 It, leaf -traces. (After Stopes; x 2.) 



practically identical in appearance; the first zone of secondary 

 xylem with its bayed inner edge consists of vertically running 

 elements; this is succeeded by a zone in which the tracheids 

 pursue a horizontal course, and beyond this second zone is another 

 band of vertical elements (fig. 581). 'Where the one zone passes 



