Trunks and branches: structure and transport

Picture the trident maple, our instructional example. Early in the growing season, you wire one young branch to bring it down to a better angle. For weeks it looks unchanged. Then you notice the bark puffing up between the coils, and the wire seems to have sunk into the branch. Nothing has snapped, and the leaves beyond the wire are a healthy green.

The wire did not tighten; the branch grew. To see why that harms the branch long before anything breaks, you need to know what a branch is made of and which parts of it are alive.

A branch from the outside in

A trunk or branch is built in layers nested like sleeves, and a cut straight across shows them as rings.

The bark: a dead shield over a living layer

Bark is everything outside the wood, and it has two very different parts. The outer bark is dead tissue, chiefly cork, added from the inside by a thin living layer called the cork cambium. Dead does not mean useless: cork is a tough, water-resistant shield against drying, injury, and disease organisms. On a young branch, it is only a thin skin.

Beneath it lies the inner bark, a layer of mostly living cells. Its main tissue is phloem, the tissue that carries sugars and other compounds made in the leaves to wherever they are needed: growing shoot tips, roots, and storage tissues.

The cambium: where thickening starts

Between the bark and the wood is the vascular cambium, a layer only a few cells thick and too thin to see unaided. While the tree is growing, its cells divide, making new wood toward the inside and new phloem toward the outside. Like the growing tips of shoots and roots, the cambium is a meristem, a group of cells whose work is making new cells, and it is why trunks and branches grow thicker.

The wood: water pipes and support

Inside the cambium is the wood, built from layer upon layer of xylem, the tissue that carries water and dissolved minerals up from the roots. Its water-conducting cells die as they mature, leaving hollow, stiff-walled cells that link into long pipelines, built a little differently in broadleaf trees and conifers (Lesson 8).

In many older trees the wood has two zones. The outer sapwood is the working part: it conducts water, and living cells threaded through it in thin lines called rays store carbohydrates. The inner heartwood no longer carries water and has no living cells. It is often darker from deposited compounds, and not every tree forms it distinctly. Both zones help hold the branch up.

Heartwood forms only as a stem ages, so a young branch, the kind usually wired, is typically all sapwood. At the very center is the pith, a small core of soft tissue left from the shoot's first year.

The living sleeve

“Inside a branch” shows a short piece of an older branch with a wedge cut out. In “Explore,” choose each layer from “Outer bark” inward to “Pith” and check what you have just read: is it alive, and what does it do? Then choose “All living tissue.” The drawing colors the cambium green only so you can find it; in life it is colorless, and the green under scratched young bark is living bark tissue outside it.

With “All living tissue” chosen, the inner bark and cambium, made mostly of living cells, light up as a thin band under the cork, drawn far thicker than life; the hatched sapwood is only partly alive. The cork, the heartwood, and often the pith are dead.

Next, switch to “Label it” and match each of the six letters to a layer. They do not run in order from the outside in, so look at where each one points.

Water up, sugars wherever they are needed

Back in “Explore,” turn on “Show transport.” Follow the solid blue arrows and dashed amber lanes on the cut faces, then find the same channels end-on as dots on the cut end.

Xylem and phloem together are the tree's vascular tissue, an unbroken transport system from the roots into every leaf. They carry two mostly separate flows. Water and dissolved minerals rise through the sapwood. Sugars travel mainly in the phloem toward wherever they are being used or stored at the time, not by gravity: down toward the roots, or up toward opening buds and growing shoot tips.

There is no single “sap” rising in the wood and returning through the bark (Lesson 8 follows the water, Lesson 9 the sugars).

How a branch grows thicker

In “Inside a branch,” turn on “Show growth” and step through the four stages with “Next,” or press “Play.” Watch where new wood and new phloem appear, and where the bark surface ends up compared with the dashed line marking its start.

The cambium adds new wood on its inner side, so the newest wood always lies close under the bark and the oldest near the center. The older wood never moves; the cambium and everything outside it are pushed outward to make room. The outer bark cannot stretch much, so as the branch widens it cracks, flakes, or peels, while the cork cambium makes new cork beneath it. That is why an old trunk's bark is often deeply furrowed while a young shoot's is smooth.

Each season's wood forms a growth ring. Early in the season the cambium makes wide, thin-walled cells, called early wood; later it makes smaller, thicker-walled late wood. The sharp line where one season's late wood meets the next season's early wood marks the edge of a ring. In many temperate trees, including the trident maple, one ring forms each growing season, so counting rings gives a rough age; rings can be faint or irregular, especially in tropical trees.

Why a hollow tree can live

The living sleeve explains a sight that surprises many people: an old tree hollowed out by decay, still leafing out every spring. Its essential living tissues were never in the middle. As long as an outer sleeve of sapwood, cambium, and bark connects roots and leaves, the tree carries on, though a hollow trunk is weaker.

When the living sleeve is damaged

When the phloem is cut. Remove or crush a complete ring of bark, and the phloem is severed all the way around: this is girdling. Sugars made by the leaves beyond the injury can no longer pass it, so the roots and trunk below lose that supply, while sugars build up on the leaf side, where the bark often swells. A girdled trunk usually kills the tree over time, as its roots starve.

When the cambium is crushed. No new wood or bark forms at that spot, and the tree does not repair it. Instead, living cambium at the edges of the injury slowly spreads new wood and bark over the wound, leaving a scar that can stay visible for years (Lesson 14).

What wire does to a growing branch

Wire holds a branch in a new position while the branch lays down new wood that helps it keep that shape. It is applied to rest snugly on the bark without pressing. But the branch keeps growing and the wire does not, so each new layer pushes the bark outward against coils that cannot give. When the wire begins to cut into the bark, the injury is called wire bite.

In “How wire bites a growing branch,” press “Next” one stage at a time or move the slider. At each stage, read the status card, then check the magnified view (A), a lengthwise cut through one coil, to see which layer the wire is pressing.

“Just applied” and “Some growth” look almost identical from outside, but by the second stage the slack is gone, and every bit of thickening becomes pressure. The first sign you can see, bark rising beside the coils at “Starting to bite,” comes when the wire is already squeezing the inner bark. Through every stage, the row headed “What still looks fine” reports an unbroken branch, green leaves, and flowing water.

The harm reaches beyond the coil. Sugars from that branch's leaves are held up at the wire, so the trunk and roots, which use them for growth and storage, receive less. Because the crushed cambium makes no new wood along the spiral, the branch thickens unevenly there and can be weaker. How fast this unfolds depends on how fast the branch thickens, not on the calendar: weeks in vigorous growth, months in slow growth. Thickening can continue long after the new shoots have stopped lengthening.

Bending strains the same tissues another way. The outside of a curve is stretched and the inside compressed; bent too far or too fast, a branch can crack, or its bark and cambium can tear from the wood. Course 10, Wiring and First Styling, and Course 15, Advanced Structural Techniques, will cover bending.