26
Science and the Garden
Fig. 1.14 Section of single vascular bundle from the stem of a Dicotyledon such as buttercup (Ranunculus spp.). Drawing by Chris Went.
which the leaves, axillary buds and branches emerge, for the vascular supply to these organs necessitates sometimes complex patterns of branching of both the phloem and the xylem tissues - a veritable 'spaghetti junction' of the plant world. Moreover, there may be cross links between the individual bundles in the internodes, the portion of the stem separating the nodes. Finally, complex repositioning of vascular bundles also occurs in the hypocotyl, the portion of the stem which at its base links with the root, for in the root proper the xylem comprises a central core, with the phloem tissues dispersed around it, as we shall see below.
Secondary thickening As the stem of a dicotyledonous plant ages in woody plants, it undergoes a process called secondary thickening (Fig. 1.15), in which woody
tissues are created to provide additional support, and an outer layer of bark is laid down to protect the phloem beneath. This process begins when the cambiums of adjacent vascular bundles link up by the development of the interfascicular cambium referred to above (Fig. 1.15b). Thus a continuous cylinder of cambial cells is created, running the whole length of the stem, like the letters through a stick of seaside rock. By the end of the first season of secondary growth, this cambium has formed a complete cylinder of xylem tissue on its internal face and a somewhat thinner layer of phloem tissue externally (Fig. 1.15c). During the next season a further cylinder of xylem is produced, so that two annual rings of xylem are visible. A second layer of phloem is also formed, but in this case the first layer of phloem is disrupted to accommodate the second. The different fates of the secondary xylem and phloem are explained by