Fig. 7.2-1. Longitudinal section of corn stem (Zea mays). This section cuts through a vascular bundle, with phloem on the right, xylem on the left. The first conducting cells formed in the xylem were vessel elements with annular secondary walls (that is, their secondary walls were in the form of several rings per cell; also called annular thickenings [notice this is “annular” – ring-like, not “annual” = yearly]). After the vessel elements completed their differentiation and died, the surrounding tissues continued to expand, stretching the primary walls of the vessel elements, thus pulling the rings farther apart. The rings in the center cell (three rings indicated by horizontal arrows) were formed much closer together but have been pulled apart. The vessel element just to the left became mature (and died) earlier, so has been subjected to more stretching and its rings are very far apart (three tilted arrows); the primary wall of this vessel element has probably been torn, so the cell can no longer conduct water.