Stems

Core Concepts

Nodes, internodes, and buds

A stem is built as a repeating module: node (leaf attachment) → internode (elongated gap) → node → internode, and so on. At each node, an axillary bud sits in the leaf axil (the angle between leaf and stem); axillary buds are dormant meristems that can grow into branches if released. The terminal (apical) bud at the stem tip contains the apical meristem that lengthens the stem and produces new leaves. Apical dominance — the suppression of axillary buds by hormones from the terminal bud — is why a single-stemmed plant becomes bushy when its tip is cut: removing the tip releases the axillary buds. This is also why pruning shapes plants.

Internal anatomy: herbaceous stems

A herbaceous eudicot stem, in cross-section, shows the three tissue systems laid out in rings: epidermis (dermal) on the outside; cortex (ground tissue) beneath it; then vascular bundles arranged in a ring, each bundle containing xylem (toward the inside) and phloem (toward the outside), often separated by a vascular cambium; and pith (ground tissue) filling the center. A monocot stem looks different: the vascular bundles are scattered through the ground tissue rather than ringed, and there is usually no distinct cortex/pith separation. Scattered versus ringed bundles is the single most reliable stem trait for distinguishing the groups — parallel leaf veins are the above-ground echo of the same pattern.

Woody stems and secondary growth

Woody stems start with the same primary structure, then add lateral meristems. The vascular cambium is a cylinder of dividing cells between xylem and phloem. Each year it produces secondary xylem (wood) to the inside and secondary phloem to the outside; because more wood is added than phloem, the trunk thickens mainly by accumulating wood. The cork cambium produces cork (part of the bark) to the outside; the bark of a mature tree is everything outside the vascular cambium — the secondary phloem plus cork layers. The visible annual rings are the wood added in one growing season: wide, thin-walled cells (earlywood, formed in spring) give way to narrower, thicker-walled cells (latewood, formed later in the season), and the boundary between one year's latewood and the next year's earlywood is the ring line. Heartwood is the older, darker, non-conducting center (often filled with resins and tannins that resist decay); sapwood is the younger, lighter, water-conducting outer wood. Counting rings gives a tree's age — and ring widths record growing conditions from drought to good years.

Modified stems

Evolution has repurposed stems into storage and spreading organs, and recognizing them is a classic exam trap because several look like roots:

• Rhizomes — horizontal underground stems (ginger, iris, many grasses). They have nodes, internodes, and scale leaves, and can send up new shoots.
• Stolons (runners) — horizontal above-ground stems that root at nodes to make new plants (strawberry runners).
• Tubers — swollen stem tips, such as the potato. The "eyes" are axillary buds, the proof that a potato is a stem, not a root.
• Bulbs — a short stem surrounded by fleshy, food-storing leaves (onion, tulip).
• Corms — short, swollen solid stems with papery leaves (crocus, gladiolus).

Stems in the human economy

Wood from secondary xylem is the most important stem product: lumber, paper pulp, and fuel. Cork (from the cork oak's cork cambium) seals wine bottles. Sugarcane is a stem whose parenchyma stores sucrose. Cinnamon is bark — the inner phloem/cortex of a tropical tree. Potatoes are tubers. Whenever you ask "is this a root or a stem?", check for nodes and buds: stems have them, roots do not.

Common Confusions