Plants grow in two directions, and each is handled by its own kind of growth. Primary growth makes a plant longer, extending roots and shoots from their growing tips. Secondary growth makes stems and roots thicker, adding wood and bark from thin rings of dividing cells inside the stem. A seedling stretching upward is showing primary growth, while a tree trunk widening over decades is showing secondary growth.
The two aren’t stages a plant passes through. A tree does both at once every year, lengthening its twigs while its trunk thickens. Nearly all plants have primary growth, but only some, mainly trees, shrubs and many other non-grass flowering plants, have true secondary growth, which is why grasses and palms never form true wood.
Where Each Kind of Growth Happens
Unlike animals, plants keep growing all their lives, and they do it from small zones of cells that never stop dividing. These zones are called meristems, and the rest of this article calls them growth zones. The ones at the tips of roots and shoots produce primary growth, pushing roots deeper and shoots higher.1Encyclopedia Britannica. meristem They also build the plant’s first, basic body: its first stem, leaves and roots.2Encyclopedia Britannica. meristem Secondary growth comes from growth zones that run lengthwise inside stems and roots, and they widen those organs instead of lengthening them.3EBSCOhost. Plant Growth
Because tip growth and side growth happen in different places, a branch can be getting longer at its end while its base gets thicker. A nail hammered into a trunk at shoulder height will still be at shoulder height years later, because trunks don’t stretch. Height is added only at the very tips of branches. The trunk around the nail keeps widening, though, and may slowly swallow it.
That’s also why a tree does not lift its lower branches as it grows. They stay put because the trunk below them never lengthens. Keep this in mind when planting near paths or driveways, since a low branch will still be low in twenty years.
A third kind of growth zone sits at the base of leaves and between leaf joints, especially in grasses.4Encyclopedia Britannica. meristem These let leaves and stems regrow from the bottom after the tips are cut or grazed.5Wikipedia. Meristem They still add length, not thickness, so they count as primary growth rather than secondary.
This is why a lawn recovers quickly after mowing: the grass keeps its growth zone near the ground, below the blade. Cut the tip off a tree shoot and you remove its tip growth zone, so that shoot stops extending. Side buds lower down then take over.
How a Trunk Gets Thicker and Makes Wood
Just under the bark of a tree lies the vascular cambium, a sheet of dividing cells only one to several cells thick.6PubMed. Vascular cambium stem cells: past, present and future The cells it makes toward the center become new wood, called xylem, which carries water up from the roots.7PubMed. Vascular cambium stem cells: past, present and future The cells it makes toward the outside become inner bark, called phloem, which carries sugars made in the leaves.8PubMed. Vascular cambium stem cells: past, present and future In each row of cambium cells, only one cell acts as the true source, able to produce both wood and inner bark.9PubMed. Vascular cambium stem cells: past, present and future
Each year’s new wood is laid down just outside the previous year’s, so the oldest wood sits at the center. The cambium and bark get pushed outward as the trunk widens. Older inner bark is crushed and stretched in the process.
The cambium never ends up buried in the center, because it stays just under the bark while its own daughter cells pile up behind it. As the trunk’s circumference grows, the cambium has to cover a bigger circle each year, so its cells also divide sideways to lengthen the sleeve itself. Without those sideways divisions, the sleeve would split as the wood core expanded.
You can see this layering by peeling back the bark on a twig. Just beneath is a thin, moist, often green layer, the living zone that holds the cambium. Inside it is pale, hard wood, the accumulated xylem. Gardeners scratch twigs to check whether a plant is alive: green and moist means living, while dry and brown underneath means that part has died.
It also explains why damage all the way around a trunk can kill a tree. A string trimmer, a tight tie or a chewing animal can strip bark and cambium in a full circle. That cuts the inner-bark route that carries sugar down to the roots, and no new wood or bark can form across the gap. The roots slowly starve, even though the leaves may look fine for a while.
Where Bark Comes From
Secondary growth uses two growth layers: the cambium and the cork cambium.10PubMed Central. Cork Development: What Lies Within Since the trunk keeps widening, the thin original skin of a young stem tears and breaks down.11PubMed Central. Cork Development: What Lies Within The cork cambium builds a new protective covering that replaces that skin in woody stems and roots.12Southern Illinois University Carbondale. Lecture 14 Periderm It makes waterproofed cork cells toward the outside and a thin layer of living cells toward the inside.13PubMed. The development of the periderm: the final frontier between a plant and its environment The outer bark you touch is mostly cork, a barrier between the tree and weather, insects and disease.14PubMed. The development of the periderm: the final frontier between a plant and its environment
You can watch this happen on a twig. A young one is green and smooth, still wearing its first thin skin. Within a season or two it turns brown and rougher as cork forms.
Cork can’t stretch, so the outer bark splits as the trunk widens underneath, and new cork forms below to keep the surface sealed. Different trees crack in their own ways, as ridges, plates or peeling strips, so bark patterns can help you identify a species.
Which Plants Have Secondary Growth
Woody plants and many herbaceous non-grass flowering plants show some secondary growth.15PubMed Central. Cork Development: What Lies Within Across seed plants, the cambium is the standard source of thickening, except in monocots.16PubMed Central. Loss of Wood Formation Genes in Monocot Genomes17PubMed Central. Loss of Wood Formation Genes in Monocot Genomes
Some garden herbs do thicken. Many annuals, such as sunflowers and tomatoes, form a little woody tissue at their base, which is why old tomato stems get tough and hard near the soil. They never build the years of wood a tree does, because they die after a season.
Monocots, such as grasses, lilies, palms and orchids, completely lack growth from a vascular cambium.18PubMed Central. Loss of Wood Formation Genes in Monocot Genomes Their wood and inner bark channels are already scattered through the stem as separate bundles, rather than arranged in a ring with a cambium able to form between them. With no continuous sleeve of dividing cells, nothing can add new wood on one side and new inner bark on the other, so the stem can’t widen year by year. A monocot that gets bigger usually does so by producing more shoots or by cells enlarging.
A palm trunk therefore reaches most of its width early. Cut palm wood shows scattered fibers, not rings, and the palm’s “bark” is often old leaf bases rather than cork.
Bamboo is a common confusion. It’s hard because its cell walls are toughened, not because it makes rings of wood. A cane emerges at nearly full width, grows tall quickly by lengthening, then stops.
A few monocots do thicken, though. The dragon tree does it through an unusual growth layer that builds strands of water-carrying cells differently from normal wood.19PubMed Central. Atypical origin, structure and arrangement of secondary tracheary elements in the stem of the monocotyledonous dragon tree, Dracaena draco Some monocots also show secondary thickening in their roots, with different root parts specialized for support or water storage.20PubMed Central. Pattern of secondary growth in monocot roots: unveiling longitudinal and cross-sectional variability
How Secondary Growth Makes Tree Rings
The cambium works on a yearly rhythm. In spring it divides rapidly, it slows in late summer, and it stops in fall.21Penn State Extension. What Exactly Are Growth Rings? So one growth ring forms each year.22Penn State Extension. What Exactly Are Growth Rings? Earlywood, made early in the season, has larger, thinner-walled cells and often looks lighter.23Plant Note. How Tree Rings Form and What Their Growth Patterns Can Tell Us Latewood, made later, has smaller, thicker-walled cells and often looks darker and denser.24Plant Note. How Tree Rings Form and What Their Growth Patterns Can Tell Us
The difference comes down to sugar. Early in the season less sugar reaches the cambium, so cells have more time to enlarge before their walls harden. Later, plentiful sugar builds thick walls fast.25PubMed Central. The Physiological Mechanisms Behind the Earlywood-To-Latewood Transition: A Process-Based Modeling Approach The sharp line between dark latewood and the next spring’s light earlywood marks each year’s boundary.
Rings are clearest in places with marked seasons.26Plant Note. How Tree Rings Form and What Their Growth Patterns Can Tell Us In tropical forests with no cold or dry season, the cambium may grow fairly steadily, so the bands can be faint or missing, and age estimates there need other methods. A narrow ring often points to a dry or stressful year.27Frontiers in Ecology and Evolution. Forest Growth Responses to Drought at Short- and Long-Term Scales in Spain: Squeezing the Stress Memory from Tree Rings
You can read this on a stump at home. Count the dark latewood bands from center to bark to estimate age, and count near the bottom, since higher up the trunk is younger. Wide rings suggest good growing years and narrow ones hard years. A lopsided center can mean the tree leaned or grew toward light.
Why the Cambium Matters for Grafting and Tree Wounds
When grafting, the cambium of the scion (the cutting) must line up closely with the cambium of the rootstock.28University of Minnesota Extension. What Is the Difference Between Primary and Secondary Growth? Matched cambium cells form a callus bridge that joins the two pieces.29University of Minnesota Extension. What Is the Difference Between Primary and Secondary Growth? With poor alignment or gaps, the link forms weakly or not at all, no new wood or inner bark grows across, and the graft fails.30University of Minnesota Extension. What Is the Difference Between Primary and Secondary Growth? So line up the green layer just under the bark on at least one side of the cut, especially when the pieces differ in thickness.31University of Minnesota Extension. What Is the Difference Between Primary and Secondary Growth?
In a whip-and-tongue or cleft graft, both pieces are cut with clean, matching slanting faces. The thin green cambium lines are pressed together and wrapped tightly. When the scion is narrower than the rootstock, set it to one side so the cambium meets on that edge. Keep the union snug and protected from drying until it knits.
Trees are wounded throughout their lives by broken branches, impacts and animals, and wounds can lead to decayed wood.32Arboriculture & Urban Forestry (AUF). Wounding, Compartmentalization, and Treatment Tradeoffs Trees confine injury within boundaries to keep the affected wood as small as possible.33Arboriculture & Urban Forestry (AUF). Wounding, Compartmentalization, and Treatment Tradeoffs They also protect wood formed after the wound from the decay that started there.34Arboriculture & Urban Forestry (AUF). Wounding, Compartmentalization, and Treatment Tradeoffs
That’s not healing the way skin heals. Skin regrows the damaged area, but trees can’t replace injured wood. New wood from the cambium grows around an injury, rolling over the wound from the edges, while the damaged wood inside stays damaged. Rot can still sit inside a trunk that looks sealed. Clean pruning cuts just outside the branch collar help the tree seal faster, and it’s worth avoiding unnecessary wounds from mowers, trimmers and careless pruning, since each leaves a permanent walled-off spot.