The structure
of a tree.
From the underground city of roots, through the working column of the trunk, into the reaching branches and the leaf — an illustrated guide to every part, and how all of them work at once.
Three things at once.
A tree is, structurally, three things doing different work at the same time. It is a column of fluid moving upward through living tissue — the trunk and its xylem, lifting water from the soil to the leaves against gravity, against everything physics seems to suggest is possible. It is a factory producing sugars in green panels exposed to light — the leaves, each one a small contract with the sun. And it is a vast hidden city in the soil — the root system, which is, by mass, often half of the whole organism.
This is a guide to all three, and to the parts that connect them.
The first thing to understand about a tree is that the part of it you can see is not, on most days, the part doing the most work. The part doing the most work is the part you cannot see at all.
The underground city.
What you can't see is, by mass, half the tree. The root system of a mature oak extends roughly 1.5 to 3 times the diameter of its canopy, going outward in a wide, shallow disc rather than the deep cone most people imagine. The deep taproot of childhood drawings exists in a few species — pine, hickory, the very young — but in most mature deciduous trees, the working roots live in the top sixty centimetres of soil. This is the layer where oxygen, water, and nutrients are richest. The tree has, sensibly, gone where the business is.
Within that disc, the architecture has hierarchy. The largest roots — the structural roots — extend like buttresses outward from the trunk, providing mechanical anchorage. They taper into smaller roots, which taper into smaller still, and finally into the absorbing roots: the fine roots, often less than two millimetres across, which carry the actual work of bringing water and minerals into the plant.
The fine roots themselves are not the smallest part. Each is covered, near its growing tip, with microscopic root hairs — single-cell extensions that multiply the absorbing surface area by orders of magnitude. A mature tree may have on the order of one hundred billion root hairs. Most of them last only a few weeks before being replaced.
And beyond the root hairs is the network most people have never heard of: the mycorrhizae. These are fungal partners that wrap themselves around or grow inside the fine roots, then extend their own filaments — hyphae, thinner than root hairs by a factor of ten — out into the soil for distances dozens or hundreds of times further than the tree itself can reach. The exchange is straightforward and ancient: the tree gives the fungus sugars produced in its leaves; the fungus gives the tree water and dissolved minerals.
This relationship is so old it predates the existence of true roots. The first land plants, four hundred million years ago, had no roots at all, only the fungal partners that did the absorbing for them. The roots came later. The partnership came first.
"In a healthy forest, fungal networks connect adjacent trees to one another — exchanging not only sugars but information. The forest is not a collection of individual trees. It is a connected organism."
The working column.
The trunk is the only part of the tree that doesn't grow longer. It just grows thicker.
This is one of those facts that, once you've internalised it, changes how you see every tree. The trunk you are looking at — the one that begins at the soil and ends in the lowest branches — was already that length when the tree was young. As the tree has aged, the trunk has added rings of new tissue around its outside, increasing in girth, but the lowest branch has stayed at exactly the height it was the year it formed. The tree has grown upward only at its tips.
Inside the trunk, from outside to inside, there are five distinct zones. The outer bark — the rough, often patterned outer surface — is dead tissue. Its job is protection: against fire, against insects, against pathogens, against the daily abrasion of weather. Different species have radically different strategies: cork oaks grow spongy fire-resistant bark; beeches grow smooth bark that lets nothing get a grip; birches peel theirs off in horizontal sheets.
Beneath the outer bark lies the inner bark, or phloem — living tissue that carries sugars downward, from the leaves where they are made to the roots and storage tissues where they are used. A tree cut by a horizontal ring through this layer will starve to death over the following months even if everything else looks fine. This is girdling.
Beneath the phloem is the cambium — a single-cell-thick layer of actively dividing tissue. Almost everything you think of as a tree's growth happens here. The cambium produces new phloem on its outside and new wood on its inside, every year. The entire visible expansion of a trunk is the work of this thin sheet of cells.
Then the sapwood (xylem) — the outer rings still functioning, dead but hollow tubes carrying water upward against gravity, sometimes a hundred metres or more. The mechanism is one of the most remarkable processes in biology: water molecules cohere so strongly that, as the leaves transpire from the top, they literally pull a continuous column of water up the entire height of the tree, the way you might pull a thread.
And at the centre, the heartwood: the inner core, no longer carrying water, no longer growing, doing nothing but providing structural support. The tree carries its own past like that. You could call it memory.
A reaching geometry.
Branches are the extensions of the trunk's vascular system into the surrounding air, and they follow rules.
Every species has a characteristic branching pattern, determined partly by genetics and partly by the tree's response to its environment. Some species — oaks, for instance — branch widely and irregularly, producing the heavy gnarled crowns one expects from oaks. Others — Lombardy poplars, certain firs — have a strong apical dominance, in which the central leader suppresses lateral growth, and the tree grows tall and narrow. Willows weep, beeches arch, plane trees fork. None of these are decorative choices. They are answers to a question every tree asks continuously: where is the light, and how do I get to it without falling over.
At every node — every junction where a branch meets a larger branch — there is a small thickening called the branch collar. The collar contains a chemical signalling system that lets the tree, in effect, decide whether a branch is worth maintaining. If a branch is in shade, or has lost too many leaves, or is for whatever reason no longer earning its keep, the tree will cease investing in it. Eventually it dies and falls. This is the tree's self-pruning, and it is the reason a healthy forest's understory is mostly clear of dead branches.
Where new branches come from is more interesting still. Along the stem of every twig are small structures called buds — undifferentiated meristematic tissue, each one a potential branch. Most never grow into anything. The tree maintains them in dormancy as insurance, in case it loses a limb and needs to replace one quickly. After a storm, or after pruning, or after fire damage, you will see a tree push out new growth from old wood. Those are dormant buds, waking up to fill a gap. They may have been waiting for fifty years.
The buds themselves are sealed structures — tiny capsules of next year's leaves and shoots, wrapped in protective scales, holding the whole next season in compressed form. Walk in the woods in winter and look closely at the buds: they are next year, already packed.
The engine.
A leaf is a solar panel and a heat exchanger and an air filter and a chemical factory at once.
Its anatomy is layered. On top, the cuticle — a waxy, transparent layer that prevents water loss. Below that, the upper epidermis, a single layer of cells. Below that, the palisade mesophyll: tall, dense, tightly packed cells, where most of the photosynthesis happens. Below the palisade, the spongy mesophyll: a looser layer with air spaces between the cells, where gases can move freely. And below the spongy layer, the lower epidermis, which contains the stomata — small pores, surrounded by two guard cells each, that open and close to admit carbon dioxide and release oxygen.
The whole structure is laced with veins: vascular bundles carrying water and minerals into the leaf from the xylem of the petiole, and sugars out of the leaf back into the phloem. The pattern of veins is itself a kind of branching, repeating in miniature the same geometric logic as the tree's larger architecture.
The pigment that does the work is chlorophyll, contained in small organelles called chloroplasts, packed densely into the palisade cells. Chlorophyll absorbs red and blue light strongly and reflects green — which is why leaves look green to us. Other pigments — carotenoids, anthocyanins — are present in smaller amounts year-round, but masked by the chlorophyll. In autumn, when the tree begins to draw nutrients back out of the leaf in preparation for winter, the chlorophyll degrades first. The yellows, oranges, and reds were there the whole time. We just couldn't see them.
The trade-off at the heart of leaf design is this: the stomata that admit CO₂ also let water out. A leaf is, in essence, a sieve that has to remain open enough for photosynthesis but closed enough to avoid dehydration. The regulation is constant. Stomata respond to light, to humidity, to internal water status, to chemical signals. This balance, summed across every leaf, is what determines a tree's annual growth.
A mature oak has somewhere on the order of two hundred thousand leaves in summer. Each is a small contract with the sun, renewed daily.
The flowering, and the seed.
Most people associate trees with leaves and forget that they also flower. Even trees that don't appear to flower do — the wind-pollinated species (oaks, pines, beeches, birches) produce small, inconspicuous flowers, often hidden among the new spring growth. The showy flowers of magnolias, cherries, and lindens are the exception; the catkin of an alder is the rule.
Reproductive strategy varies enormously. Conifers — pines, spruces, firs — produce cones rather than flowers: separate male and female structures, with the male cones producing pollen that drifts through the spring air sometimes for hundreds of kilometres. The female cones, more familiar to non-botanists, develop into the woody seed-bearing structures we recognise. The seeds inside are typically winged, dispersed by wind.
Flowering plants — the angiosperms — encase their seeds in fruit, broadly construed. An acorn is a fruit. A maple samara is a fruit. A cherry is a fruit. The variation in fruit type reflects the variation in dispersal strategy. Wind-dispersed seeds are light, winged, released in autumn. Animal-dispersed seeds are inside something delicious, designed to be eaten and the seed defecated elsewhere. Squirrel-cached acorns are designed to be planted by being forgotten.
Each seed contains, in a small dehydrated package, the embryo of an entire new tree, along with enough food for the first few weeks of growth. Many can wait, dormant, for years before germinating. Some species require specific conditions: fire (jack pine), passage through an animal gut (certain palms), specific temperature cycles. The seed is patient. The seed is the tree's bet on the future.
Time, made visible.
If you cut a tree and look at the cross-section, you see rings. Each ring is one year.
The boundary between rings — the dark thin band — is the late-season wood, called latewood. It is denser, with smaller cells, formed when the cambium is slowing down toward the end of the growing season. The lighter, thicker band inside each ring is the earlywood: larger cells, more porous, formed in spring when growth is most active.
What you can read in rings depends on how carefully you look. Wide rings mean the tree had a good year — adequate water, warm temperatures, plenty of sun. Narrow rings mean stress — drought, cold, defoliation by insects, a year of poor performance for whatever reason. A burn scar leaves a permanent mark: a curved black or grey crescent where the cambium died on one side and was overgrown later from the other.
By cross-referencing the ring patterns of timbers from different times and places — a technique called dendrochronology — scientists can date wood with year-level precision going back thousands of years. Buildings, ships, mummified corpses, ancient instruments: all can be dated this way, given enough rings to work with. The bristlecone pine of the American Southwest has provided a continuous, overlapping ring record going back nearly nine thousand years.
A tree, then, is also a clock. It is the simplest historical instrument that exists.
The whole tree, working at once.
A tree is not a collection of its parts. It is an integrated organism doing many things simultaneously, and almost continuously.
On a typical morning in midsummer, all of the following are happening at the same time. The roots are absorbing water and dissolved minerals from the soil, using both their own surface area and the much greater surface area of their fungal partners. The xylem is moving that water upward at speeds ranging from a few centimetres to a few metres per hour, depending on species and conditions. The leaves are transpiring water out through their stomata; that loss of water at the top is the engine that pulls the column upward. Simultaneously, those same leaves are absorbing carbon dioxide from the air, combining it with the water and sunlight in the photosynthesis reaction, and producing sugars. The phloem is moving those sugars down through the inner bark, distributing them to where they are needed: to the growing tips, to the cambium, to storage tissues in the trunk, and to the mycorrhizal partners that helped pay for them in the first place.
While all of this is going on, the tree is also defending itself. It is producing tannins, terpenes, and other secondary compounds that taste bad to herbivores and inhibit fungal infection. If a branch is wounded, the cambium will compartmentalise the wound by laying down a barrier of dense, chemically resistant wood. If a pest is detected, chemical signals may be released that warn neighbouring trees, via the mycorrhizal network, that defences should be raised. The tree is regulating its temperature through transpiration. It is sensing day length and adjusting its hormones accordingly. In the spring, it is unfolding pre-formed leaves and pushing new buds from dormancy. In the autumn, it is reabsorbing nutrients from the leaves before letting them fall.
About seventy percent of the sugars a tree produces by photosynthesis are used for its own respiration and growth. The remaining thirty percent are exported downward, much of it to the roots and the fungal partners. Roughly that ratio applies to nearly every tree, of nearly every species, on nearly every continent.
There is a common misunderstanding worth correcting here: trees do not only produce oxygen. They also consume it. Every living cell in the tree — in the roots, in the cambium, in the buds, in the still-living parts of the trunk — respires continuously, twenty-four hours a day, taking in oxygen and releasing carbon dioxide, just like an animal. The difference is that during daylight hours, when the leaves are photosynthesising, the rate of oxygen production far exceeds the rate of oxygen consumption, so the net flow is outward. At night, with no photosynthesis happening, the net flow reverses. Trees breathe both directions. The forest, on a still summer night, is gently exhaling CO₂.
The water moving up the trunk is, in volume, also surprising. A single large oak can move several hundred litres of water from soil to atmosphere on a hot summer day — enough to fill a bathtub, daily, through its own tissues. Multiply that by every tree in a hectare of mature forest and you arrive at the figures from the field guide on preservation: tens of millions of litres of water, per square kilometre, per year, moved purely by the cohesion of water molecules pulling against the tension created by leaf-surface evaporation. It is one of the slowest, quietest, and most consequential industrial processes on Earth, and it has no moving parts, no fuel input, and no exhaust. Just sun, water, and a particular arrangement of cells.
It is, when you sit with it, an astonishing arrangement.
A closing observation.
There is no good shorthand for what a tree is. It is not a plant in the way a daisy is a plant. It is not an animal, but it senses, signals, and responds. It is not still, exactly — it moves slowly, but it moves, growing, swaying, dropping leaves, opening buds. It is not solitary. Most trees in a healthy forest are connected to neighbours through fungal networks, and exchange resources in ways that complicate the boundary between organisms.
It is, perhaps, simplest to say that a tree is a particular kind of patience. It is a strategy for capturing sunlight, drawing up water, and surviving, over the centuries, in a place that does not move. Everything about its structure is in service of that strategy. Every leaf, every ring, every root tip, every bud. The whole thing — the entire visible and invisible architecture — is the slow technical answer to a question every seedling asks once: how do I stay here, and grow, for as long as I can.
When you look at a tree, you are looking at the answer to that question, refined over four hundred million years.
The next time you walk past one, look at it for an extra minute. There is more going on than meets the eye, mostly underneath.