Fig.
7.2-2. Magnification of corn vessel element with annular
secondary walls. These secondary walls are very far apart because the
vessel element was stretched by the surrounding tissues after the element was
mature. It is difficult to tell if the primary wall of this vessel element is
still intact: if it were definitely functional, all the rings should be parallel
to each other, having the same tilt. In histological slides of mature stems,
vessel elements with annular secondary walls have often been torn apart by
internode expansion.