A sapling puts on height every spring. A tree that has been standing for a century is still getting wider, one thin layer of wood at a time. The growth that makes it wider runs from a ring of cells you could slice with a knife, and everything the ring produces is either wood or bark.
This post follows that ring, from the cells that divide to the trunk that ends up in a lumberyard.
Two Meristems, Not One
Primary growth happens at the tips of stems and roots. The cells there divide and the plant gets longer. Nothing adds thickness, so a plant that only did primary growth would stay a thin stalk no matter how long it lived.
Thickness comes from somewhere else. Two lateral meristems, both rings of dividing cells, do that work. Meristem cells are undifferentiated, so they can divide and then become specialized, and a ring of them keeps producing new tissue for as long as it lives.
- Vascular cambium produces secondary xylem inwards and secondary phloem outwards.
- Cork cambium replaces the epidermis, which secondary growth bursts open again and again.
The cork cambium exists because of a mechanical problem. Every year the trunk gets wider, which tears the original skin. Without a second meristem the plant would strip itself bare. The cork cambium supplies fresh corky cells, and over many years they build up into a layer of cork.
The Vascular Cambium Is a Ring
The vascular cambium is the main growth tissue in woody dicots and in gymnosperms. In section it looks like a ring. In three dimensions it is a tube, one cell thick, running the length of the stem and the root.
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| In a young stem the bundles are still separate, like beads on a string. A cambium ring has not formed yet — Image: Jon Houseman, CC BY-SA 4.0, via Wikimedia Commons |
The critical point is that this ring is not a plumbing structure. It moves no water, no minerals, and no sugar. It makes tissue, and other tissue does the moving. Botanists also call it the main cambium, the wood cambium, or the bifacial cambium.
How a broken ring becomes a complete one
In a herbaceous dicot the vascular bundles are separate, scattered like beads on a necklace. Each bundle has its own inner cambium, called the fascicular cambium. Between the bundles there is nothing dividing.
Then the medullary rays, which run between neighboring bundles, turn meristematic. They form the interfascicular cambium. The two kinds join up, and a continuous ring now separates the primary xylem from the primary phloem. From that point on the stem thickens properly.
That is also why grafting works. When a shoot is joined to a root stock, the two vascular cambia have to line up. If they meet, they fuse and the graft takes. If they are rotated out of alignment, it fails.
Inside, outside, and how much
Once the ring exists it works strictly in one direction. Secondary xylem is laid down on the inside, toward the pith. Secondary phloem is laid down on the outside, toward the bark. The primary xylem and phloem get pushed apart and end up buried, and the new xylem is what we call wood.
More xylem is produced than phloem, by a wide margin. A trunk is mostly wood, with a thin band of living inner bark wrapped around it.
The cambium itself is only two kinds of cell. Fusiform initials are tall and oriented along the axis of the stem, and they build the conducting tissue. Ray initials are smaller and round to angular, and they build the radial sheets that store and move material sideways across the trunk.
Six hormones regulate all of this, among them auxin, ethylene, gibberellins, cytokinins and abscisic acid. Auxin has been shown to stimulate cambial mitosis: applying it to the flat surface of a tree stump lets decapitated shoots carry on making secondary tissue.
The Cork Cambium and the Periderm
The cork cambium is also called the phellogen. It makes a three-layer package called the periderm, and each layer runs in a different direction.
| Layer | Where it forms | What it is |
| Phelloderm | inwards from the cambium | living parenchyma cells |
| Phellogen | the cambium itself | the dividing layer that makes the other two |
| Phellem (cork) | outwards from the cambium | dead at maturity, air-filled cells |
Cork cells are dead when they are finished, and full of air. That is what makes cork a good insulator and a decent barrier to gas and water. Its texture varies enormously with species and with age, which is why bark can be smooth, fissured, scaly, or flaking off.
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| A thin dividing layer with dead cork cells outside it and living tissue inside — Image: Kje4532, CC BY-SA 4.0, via Wikimedia Commons |
The bark of an old tree is not one thing. Everything outside the vascular cambium counts as bark. That includes the living inner bark, the periderm, and all the dead material piled up outside it. In a mature tree the dead outer layer has its own name, the rhytidome.
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| Years of cork cambium activity produce the loose outer sheets — Image: Rosser1954, public domain, via Wikimedia Commons |
Annual Rings and the Wood They Make
A tree does not grow evenly through the year. Growth is fast in early season and slow later on, and the difference is visible in the wood.
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| Wide and pale against narrow and dark. The contrast is what makes a ring visible — Image: Ninnel, CC BY-SA 4.0, via Wikimedia Commons |
- Early wood, also called spring wood. Forms early in the growing season, when growth is fast. The cells are large and the wood is less dense, so it looks pale.
- Late wood, also called summer wood. Forms later, when growth slows. The cells are smaller and the wood is denser, so it looks dark.
A good year with plenty of water and a long season leaves a wide ring. A drought year leaves a narrow one. Read a trunk from the center outward and you are reading a weather record.
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| Each ring is roughly one year. The newest sits next to the bark — Image: Arpingstone, public domain, via Wikimedia Commons |
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| The pale center is heartwood, dead and no longer conducting. The darker outer band is the living sapwood — Image: Roger Culos, CC BY-SA 4.0, via Wikimedia Commons |
Reading rings is harder than it looks
The neat rule of one ring per year is not exact. A tree stressed in midsummer can lay down a false ring, so one year may show several. Some species also skip years entirely. Missing rings are rare in oak and elm, which is partly why those two are so useful for dating.
Tree-ring dating works by matching patterns. Trees growing in the same region under the same climate produce rings of the same relative width in the same years. A wide ring in one tree can so be lined up with a wide ring in another.
Overlapping many trees this way builds a regional record. The method is called cross-dating, and it has produced oak and pine chronologies for central Europe running back more than 12,000 years.
Girdling: Why the Bark Is the Living Part
Remove a ring of bark all the way around a trunk and the tree dies. It is a slow way to kill something, and it shows exactly how little the wood alone can do.
Girdling cuts the phloem. The xylem is untouched, so the roots keep taking up water and the trunk keeps carrying water upward for a while. But the sugar made in the leaves can no longer travel down to the roots. The roots run out of fuel, stop producing ATP, and die first.
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| The xylem is still carrying water. What was cut was the sugar pipeline back down — Image: Rosser1954, CC BY-SA 3.0, via Wikimedia Commons |
Growers use this deliberately. Girdling a branch leaves the sugar made by its leaves with nowhere to go except the fruit, so the fruit sets and grows larger and sweeter. Grape growers call the same technique cincturing, and it is used on avocado, apple, mango, citrus and litchi as well. A tree normally heals in four to five weeks.
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| Sapsuckers drill rings of holes for the sap, which girdles the tree by accident — Image: Pompilid, CC BY-SA 3.0, via Wikimedia Commons |
Cork Can Be Harvested Without Killing the Tree
Most bark protects a tree by being part of it. Cork oak is the exception, because the cork cambium sits deeper than the layer that gets taken. Strip the outer bark and the tree grows a new cork cambium further in, then another layer of cork outside it.
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| Stripped pale wood, with the tree still alive and already regrowing cork — Image: Alex Lomas, CC BY 2.0, via Wikimedia Commons |
That single property is why cork ends up in bottle stoppers, floor tiles, gaskets, and the fairings of wind-tunnel models and rocket nozzles. A material light enough to float, stiff enough to seal, and that regrows on the tree.
Which Plants Do This, and Which Do Not
Secondary growth is normal in dicots and gymnosperms. It also happens in plants nobody would call woody, including tomato, potato tuber, carrot taproot and sweet potato. A few long-lived leaves manage it too.
Monocots are the exception, and the exception is instructive. Their ancestors lost the vascular cambium, and the arrangement of tissue in the stem cannot be rebuilt without changes that are not going to happen.
- Palms thicken by dividing and enlarging parenchyma cells instead. No secondary xylem or phloem is made. This is sometimes called primary gigantism and sometimes diffuse secondary growth.
- Yucca and Dracaena do form a cambium, but a different one. It produces vascular bundles and parenchyma inside, and only parenchyma outside.
- Isoetes is the one living pteridophyte where secondary growth has been recorded at all.
The vascular cambium is absent from all seed plants apart from five angiosperm lineages that lost it independently. Those five are the water lilies, Ceratophyllum, lotus, Podostemaceae, and the monocots.
The Whole Process in One Table
| Stage | What happens | Result |
| Primary growth | division at stem and root tips | the plant gets longer |
| Cambium forms | fascicular and interfascicular cambia join | a continuous ring |
| Vascular cambium works inward | makes secondary xylem toward the pith | wood |
| Vascular cambium works outward | makes secondary phloem toward the bark | inner bark |
| Cork cambium works outward | makes dead cork cells | outer protection |
| Seasonal change | fast early growth, slower later | a visible annual ring |
| Repeated years | rings accumulate and bark builds up | a trunk with a readable history |
Key Takeaways
- Secondary growth thickens a stem or root. It is done by two lateral meristems, not by the growth tips.
- The vascular cambium is a ring in section and a tube in three dimensions. It makes xylem inwards and phloem outwards.
- The cambium transports nothing. It builds tissue, and the tissue does the moving.
- A complete ring forms when the fascicular cambium inside each bundle joins the interfascicular cambium that grew between them.
- The cork cambium, or phellogen, makes the periderm: phelloderm inwards, cork outwards, itself between.
- Cork cells are dead and air-filled. That makes cork an insulator and a gas barrier.
- More secondary xylem is made than phloem, which is why a trunk is mostly wood and only thinly wrapped in living bark.
- One ring per year is a good rule, not an exact one. False rings and missing rings both happen.
- Girdling kills a tree by cutting the phloem, not the xylem. The water keeps moving up while the roots starve.
Frequently Asked Questions
What is the difference between primary and secondary growth?
Primary growth adds length and happens at the tips of stems and roots. Secondary growth adds girth and happens in lateral meristems along the length of the stem and root.
Does the vascular cambium carry water?
No. It is a manufacturing tissue, not a conducting one. The xylem it produces carries water and the phloem it produces carries sugar. It is also called the wood cambium, which is a name about what it makes, not what it transports.
Why do palm trunks get thick if they have no cambium?
Palms thicken by dividing and enlarging the parenchyma cells already present. No secondary xylem or phloem is produced, so the growth is a different process wearing a similar result.
What happens if you cut a ring around a trunk?
The tree dies, slowly, and the phloem is the reason. Water can still rise through the untouched xylem, but sugar cannot travel down to the roots. The roots starve first, and the rest of the tree fails after them.
How old is a tree if it has 80 rings?
Probably close to 80 years, but count carefully. A stressed tree can produce more than one ring in a year, and some species skip years altogether. Professionals cross-date samples against a regional record instead of trusting a single count.
Why does bark look different on different trees?
The cork cambium has been adding layers for different lengths of time in different species, and the cells differ in how they break away. Bark can be smooth, fissured, scaly, tiled, or flaky, and one tree can show more than one texture as it ages.
Can secondary growth happen in a plant with no wood?
Yes. Tomato, potato tuber, carrot taproot and sweet potato all show secondary growth, and they are not woody plants. Any plant with a functional vascular cambium can thicken, whether or not the result looks like a tree.
Sources: Wikipedia articles on secondary growth, vascular cambium, cork cambium, periderm, wood, annual ring, dendrochronology, bark and girdling. Images: Wikimedia Commons, with authors and licenses noted in each caption.
