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.