The boundary between primary and secondary xylem is conspicuous in the distribution of tracheid diameters, which are
narrower in the primary xylem and wider in the secondary xylem ( Fig. 3A ). Additionally, in cross sections, the boundary is marked by a transition from smaller, thinner-walled, rounder tracheids to larger, rectangular tracheids ( Fig. 2D, E ). In longitudinal sections we see a transition between the characteristically of the larger-diameter secondary xylem tracheids, through a few narrower tracheids with multiaperturate scalariform bordered pits (peripheral metaxylem), to oval bordered pits of the primary xylem ( Fig. 2B ).