Fig. 16.2-3a and b. Radial longitudinal section of linden (Tilia). Notice that all the sieve plates visible here are located at about the same level, between the two lines. Just as a radial section of wood shows vessel element perforation plates aligned, a radial section of secondary phloem shows aligned sieve plates for the same reason – these sieve tube members were produced by one set of fusiform initials, and the ends of the sieve tube members here are aligned with the ends of the initials that produced them.