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.