Plant stems function in various capacities, primarily in nutrient transport and physical support. The vascular system of plants, complete with xylem and phloem, fills both purposes. Stems, along with roots, also store food for the plant. Pith, a tissue that lies in the center of the stem (or, as it is called in trees, the trunk), is responsible for some nutrient storage as well.
The vascular system in the stem is continuous with that of the roots, providing an easy route for water and nutrients to flow throughout the plant body. The organization of vascular tissues is different for various types of plants--from the scattered vascular bundles (containing both xylem and phloem) of monocots to the more orderly ring formation found in dicots. This difference in the organization of the vascular tissues has implications for the way the plant grows. (For a general discussion of monocots and dicots, see Plant Classification, Monocots and Plant Classification, Dicots.)
In monocots, xylem and phloem are organized in vascular bundles scattered throughout the stem. As the plant grows, monocot stems generate new vascular bundles for the new tissue. Monocot stems in general possess a simpler arrangement than that found in dicots; the main elements of the stem are merely the vascular bundles and the pith (used for nutrient storage) that surrounds them.
The vascular system found in dicots is somewhat more complex than that found in monocots. In the dicot stem, the vascular bundles are arranged in a ring, with pith concentrated at the core of the stem, rather than being scattered throughout the plant interior. In each vascular bundle, the xylem and phloem are separated by a substance called vascular cambium. As the plant grows, existing bundles grow larger (rather than new vascular bundles being generated, as in monocots). The vascular cambium operates by producing new xylem and phloem cells, which in turn pushes the old cells outward and forces the bundle to grow.
Within the class of plants known as dicots, herbaceous dicots and woody dicots have different arrangements of vascular tissues. In herbaceous dicots (plants, mostly annuals, with soft, non-woody stems), vascular tissue remains in discrete bundles even at maturity. In contrast, when mature woody dicots (plants, mostly perennials, with woody stems) reach maturity, the vascular bundles join together to form continuous rings around the interior of the stem. In addition, woody dicots develop a lateral system of nutrient transport in which vessels called rays extend horizontally from the phloem to the pith, carrying nutrients to the pith for storage or to the phloem for dispersal.
In addition to distributing nutrients, vascular tissues also provide structural support. In fact, the material commonly known as "wood" is actually xylem. After a time, the xylem at the center of older trees (woody dicots) ceases to function in transport and takes on a purely supportive role; this nonfunctioning xylem is called heartwood. The newer xylem closer to the surface remains active in the vascular system, and is called sapwood.