17.5: Stems are the backbone of the plant.

When an organism must reach toward the sun for the energy to make its own food, gravity can be the enemy. Plants compete with each other, and taller plants can leave shorter plants in the shade, with reduced access to sunlight. The body of a plant is one continuous structure, with the root system below ground and a portion called the shoot system above ground. The shoot is divided into two parts: stems and leaves. In some species, the shoot system also includes flowers and fruits, the reproductive structures, which we discuss in Chapter 18.

Serving as the backbone of the plant, the stem provides structural support for the plant. And in conjunction with the roots, the stem enables the plant to grow and orient its leaves toward the sun. The stem’s third function is housing the plant’s transport infrastructure—the xylem and the phloem—through which water, sugars, and other nutrients are distributed throughout the plant (FIGURE 17-13).

In plants, growth occurs in regions called meristems, made up of undifferentiated cells. These cells divide, often at very high rates, generating additional meristem cells along with cells that differentiate into epidermis, vascular tissue, or ground tissue. At the tip of a root or stem, the meristem is called an apical meristem, and division of cells there causes the root or stem to increase in length. Lateral meristems, on the other hand, consist of a layer of cells, called cambium, that cause a stem or root to become thicker, as opposed to longer, when they divide. We discuss plant growth in more detail in Chapter 18.

Stem growth and root growth are similar, except that stem growth has the added complexity that it is segmented (FIGURE 17-14). That is, the stem grows a bit and then produces nodes, little lumps of tissue from which leaves, flowers, cones, or additional stems (or even a branch) may grow.

The stems of flowering plants contain vascular bundles. (Recall that there are two types of pipelines through which sap, containing essential fluids and chemicals, is transported: water and minerals move through the xylem, and sugars move through the phloem.) As we saw in Section 17-2, these bundles are arranged in one of two distinct patterns. In eudicots, a ring of vascular bundles is arranged near the outside of the stem. At any point that you slice a cross section of the stem, you will see a ring of bundles, like a clock face. Where each of the numbers might be, there is a double pipeline. The outer pipe is the phloem and the inner pipe is the xylem. This ring of vascular bundles divides the stem’s ground tissue into two distinct areas: one inside and one outside the ring. The inner tissue is the pith and the outer tissue is the cortex. In monocots, such as corn, the vascular bundles seem less organized: they are dispersed throughout the stem. In all vascular plants, because each node of the stem may be the site of a leaf or a branch, some of the vascular bundles turn outward at this point and enter the leaf or branch.

If we started slicing up the stems of all the plants around us, it would quickly become clear that there isn’t one single design for all stems. Although the stems of all flowering plants are variations on the two themes described here, they vary tremendously (FIGURE 17-15). Most of the bulbs that gardeners plant each year, such as those for daffodils and lilies, are stems. Many modified leaves are attached to the bulb stems, forming the bulk of the bulb, and, after planting, the roots emerge from the bottom.

Some of our favorite foods come from modified stem tissue. Asparagus spears, for example, are fleshy stems. If you look closely, you can see what appear to be scales along the spear and at the tip. These are actually very tiny leaves that haven’t branched off very far from the stem. Broccoli and cauliflower also include modified stem. Also, the horizontal runners of a strawberry plant are stems, from which roots emerge at regular intervals to anchor the plant and stems to the ground. And growing underground, potatoes, too, are highly modified branching stems—not roots. When potatoes sprout, the “eyes” contain lateral buds that branch out. Similar-appearing foods that are true roots, such as sweet potatoes, never form such “eyes.”

As stems grow and branch, they increase in girth and length, becoming stronger and better able to position leaves where they can have adequate exposure to the sun for making food through photosynthesis. In the next chapter, we investigate the process by which stems develop into tree trunks and, in doing so, produce wood—perhaps the most important non-food plant product on earth.