Stems and Vascular Tissue: Xylem, Phloem and Wood

A trunk is a record book

If you count the rings on a stump, you can tell how old the tree was. If some rings are wide and others thin, you can tell which years were good for growing and which were dry. A stem is not just a stick that holds leaves up. It is a two-way highway, a storage tank, and a diary of the plant's life.

Stems have four jobs: support leaves and flowers, transport water and sugar between roots and leaves, store food and water, and make new tissue at buds and cambium. Some green stems also photosynthesize.

The outside of a stem

Look at a twig from a maple, lilac or apple tree in winter:

• The terminal bud at the tip holds next year's shoot, tightly wrapped in bud scales.
• Nodes are where leaves were attached. Each node has a leaf scar (where last year's leaf fell off) and a lateral bud just above it, which can grow into a branch or a flower.
• Internodes are the stem sections between nodes.
• Bud scale scars are rings of tiny scars left when the terminal bud's scales fell off in spring. The distance between two sets of bud scale scars is one year's growth.
• Lenticels are small bumps or dashes on the bark: pores for gas exchange.

Inside a herbaceous dicot stem

A young, non-woody dicot stem (for example, a sunflower or bean) has:

• Epidermis with a waxy cuticle, to protect and reduce water loss.
• Cortex: ground tissue, often with collenchyma for flexible support near the outside, and parenchyma for storage.
• A ring of vascular bundles. In each bundle, phloem faces the outside, xylem faces the centre, and a thin layer of vascular cambium lies between them.
• Pith: parenchyma in the centre, for storage.

In a monocot stem (corn, grasses), the bundles are scattered and there is no cambium, so most monocots don't become woody. You will compare them fully in Lesson 8.

The cells of xylem

Xylem carries water and dissolved minerals upward. Its conducting cells are dead when they work: the cell contents break down, leaving a hollow tube of thick cell wall strengthened with lignin, a tough polymer that also makes wood hard.

• Tracheids are long and narrow with tapered ends. Water passes from one tracheid to the next through pits, thin spots in the walls. Conifers such as pines and spruces have xylem made almost entirely of tracheids.
• Vessel elements are shorter and wider, and their end walls are partly or completely open. They stack end to end to form long tubes called vessels, which carry water faster. Most flowering plants have vessels as well as tracheids.
• Xylem also has fibres for strength and some living parenchyma cells.

The cells of phloem

Phloem carries sugars (mostly sucrose) and other dissolved organic molecules both up and down. Its conducting cells are alive.

• Sieve tube elements stack end to end. When mature they have no nucleus and very little inside, which leaves room for flow. Their end walls are sieve plates, full of pores.
• Each one has a companion cell beside it, joined by many plasmodesmata. The companion cell has a nucleus and many mitochondria and keeps the sieve tube element alive and loaded with sugar.
• Phloem also contains fibres (flax fibres, used to make linen, are phloem fibres).

Secondary growth: how trees get thicker

In woody plants, the vascular cambium forms a complete cylinder. Each growing season it divides:

• toward the inside, it makes secondary xylem, which is wood;
• toward the outside, it makes secondary phloem.

As the trunk widens, the epidermis splits. A second lateral meristem, the cork cambium, makes cork cells with waxy walls that become the protective outer bark. Bark is everything outside the vascular cambium: the living inner bark (secondary phloem) and the dead outer bark (cork).

Annual rings. In spring, when water is plentiful, the cambium makes large, thin-walled xylem cells (earlywood, light in colour). Later in summer it makes small, thick-walled cells (latewood, darker). The sharp line between one year's latewood and the next year's earlywood marks one year. Wide rings mean good growing years; narrow rings mean drought, cold or crowding. Scientists who study tree rings to date wood and reconstruct past climate are called dendrochronologists.

Sapwood and heartwood. Only the outer, younger rings (sapwood) carry water. The older central wood (heartwood) is filled with resins and other compounds, is often darker, no longer conducts water, and supports the tree.

Girdling. If an animal or a person strips a complete ring of bark from a trunk, the phloem is cut. Sugar from the leaves can no longer reach the roots, the roots slowly starve, and the tree dies, sometimes a year or more later. Hungry mice, rabbits and porcupines sometimes girdle young trees in winter, which is why orchard trees often wear plastic guards.