Hello again. In the previous lesson, you identified the visible regions of a tree: crown, trunk, bark, and roots. We now move beneath the trunk’s surface. A stem cross-section—such as a stump or a naturally fallen branch cut cleanly across—reveals a set of nested tissues that protect the tree, transport water and sugars, add new girth, and support the mature trunk.
By the end of this lesson, you should be able to locate bark, phloem, vascular cambium, sapwood, heartwood, and pith from the outside of a cross-section toward its center, and give the main role of each.
Using a cross-section as a map
A trunk cross-section is not simply “wood surrounded by bark.” It is a living system arranged in layers. Start at the outside edge and work inward. This is the most reliable way to identify the tissues.
The diagram also labels earlywood, latewood, growth rings, and rays. These are useful features, but for this lesson keep your attention on the six major tissues.
Here is the essential outside-to-center order:
| Position | Tissue | Main role |
|---|---|---|
| Exterior | Bark | Protects the trunk’s vulnerable living tissues |
| Just inside bark | Phloem | Distributes sugars and other organic materials |
| Very thin boundary | Vascular cambium | Produces new phloem and xylem as the stem widens |
| Outer wood | Sapwood | Conducts water and dissolved minerals, with some storage |
| Inner wood | Heartwood | Provides strong structural support; no longer conducts water |
| Center | Pith | Central tissue formed early in the stem; chiefly a storage region in young stems |
The layers do not always appear as perfectly even circles. A trunk may grow unevenly because of leaning, injury, competition, wind, or variation in light and water. The spatial order, however, remains a powerful guide.
Bark and phloem: protection outside, food distribution just within
In ordinary outdoor language, bark means the rough, visible outer surface of a trunk. Its chief purpose is protection. It limits damage from drying, temperature swings, physical impacts, insects, and disease organisms.
More precisely, botanists often use bark for all tissues outside the vascular cambium. This includes an outer, mostly dead protective layer and an inner, living region. In a cross-section, it is useful to distinguish them:
- Outer bark is largely made of dead, corky tissue. It is the cracked, flaky, ridged, smooth, or papery surface you see on a living tree.
- Phloem, often called inner bark, is living tissue nearer the cambium.
Phloem carries sugars made in leaves to places where they are needed or stored: developing roots, buds, fruits, seeds, growing shoots, and storage tissues. The material travelling in phloem is often called sap, but it is important not to confuse it with the water-rich flow in xylem.
Phloem movement is not best understood as simply “downward.” Leaves are often a source of sugars and roots are often a destination, so movement commonly goes downward in a trunk. But phloem moves sugars from sources to sinks, wherever those happen to be. For example, stored resources can move upward to expanding buds in spring.
Read the following overview before continuing. It establishes the layer order and connects the protective bark with the two main transport systems.
What is a Tree? | Forestry - USU Extension
This concise USU Extension reading introduces the layers of a woody stem and explains why the living transport tissues lie immediately beneath the outer bark.
In the introductory trunk-anatomy discussion, begin with the outside-in tour. Focus on the distinction between dead outer bark and living inner bark or phloem, then notice the cambium's location just outside the wood. Next, read the later paragraph beginning the sapwood-heartwood transition. Focus on why the outer wood remains involved in water transport while central wood becomes heartwood. You can skip the photographs and the following discussion of whether particular plants count as trees.
A practical implication follows from this arrangement: severe damage that removes bark all the way around a trunk can be lethal. Such damage can destroy phloem and cambium, cutting off the distribution of sugars to roots and preventing the tree from making new conducting tissue in that area. For observation, use an existing stump, a cut log, or a fallen branch—never cut into a living tree.
The vascular cambium: a thin layer that makes a trunk wider
The vascular cambium is a very thin, continuously dividing layer of cells between phloem and xylem. It is usually too thin to pick out clearly without magnification, even on a fresh cut surface. In the diagram, locate it as the narrow ring just inside the phloem and just outside the sapwood.
Its role is unusually important for such a narrow tissue: it produces the tissues that allow a tree to thicken.
As the cambium divides during growth:
- Cells made on the inside develop into new secondary xylem, or wood.
- Cells made on the outside develop into new secondary phloem.
- Older outer phloem becomes compressed and is eventually incorporated into the outer bark.
Because the cambium makes more xylem than phloem, wood accumulates far more visibly than phloem. This is why a large trunk is mostly xylem, even though both xylem and phloem are essential transport tissues.
The brief Aalto University video gives a helpful visual account of this boundary layer and the materials on either side of it.
In “Wood anatomy (6) cambium,” Aalto University - Wood Science uses a drawn cross-section to show where cambium sits and what it produces.
Watch the layer overview to orient yourself: visible growth rings are xylem, while cambium, phloem, and bark lie progressively farther outward. Then watch cambium in action. Focus on the cambium producing new tissue on both sides, with xylem conducting water and phloem distributing photosynthetic products.
This thickening process is called secondary growth. You will return to it later when studying how annual rings form and how trees grow through time. For now, retain the essential location and function: cambium sits between phloem and wood, making new phloem outward and new xylem inward.
Sapwood and heartwood: two functional zones of xylem
Everything inside the vascular cambium is xylem, commonly called wood. Yet not all wood performs the same job. The xylem of a mature trunk typically has an outer functional zone, sapwood, and an inner support zone, heartwood.
Sapwood
Sapwood is the younger, outer xylem immediately inside the cambium. Its main job is to conduct water and dissolved mineral nutrients from the roots toward the leaves. In many trees, sapwood also participates in storage.
Sapwood is often lighter in color than heartwood, but color alone is not a definitive test. In some species the difference is subtle, and in small branches almost all visible wood may function as sapwood.
Although sapwood is often described as “living wood,” be precise about what that means. The water-conducting xylem vessels or tracheids are generally dead when mature; their hollow walls form an efficient pathway for water. However, sapwood also contains living cells involved in storage and maintenance. Thus sapwood is a physiologically active wood zone, not a solid mass of living cells.
Heartwood
Heartwood is the older, inner xylem. As a trunk ages, its central xylem no longer needs to carry water. Those older conducting pathways stop functioning, and the wood becomes heartwood.
Heartwood’s principal role is structural support. It provides a strong central column that helps the tree support its own mass, the crown, wind loads, snow, and gravity. In many species, heartwood contains chemical compounds that make it more resistant to decay. These compounds often make it darker than sapwood, as in the diagram, but heartwood is not always visibly dark.
A crucial distinction:
Heartwood is dead tissue, but heartwood in a living tree is not evidence that the tree itself is dead or unhealthy.
A healthy old tree may contain a large core of heartwood while its thin outer zones of sapwood, cambium, and phloem keep it functioning.
The transition from sapwood to heartwood also explains why a tree does not need every annual ring to move water. The active conducting zone is usually nearer the outside, close to the cambium where new xylem is being added.
Pith: the center, not the main support column
At the very center is the pith. It formed early, when the stem was young and growing in length. Pith is typically soft and spongy compared with the surrounding wood, and it is made chiefly of storage tissue.
In a young twig, pith can be quite obvious. It may appear as a light central core, and in some species it is chambered or becomes hollow with age. In a mature trunk, it is often tiny compared with the surrounding wood and can be difficult to see.
Its main role is therefore best stated as:
- Pith stores materials and occupies the central tissue of a young stem.
Do not mistake the pith for heartwood. Pith is the original center of the stem; heartwood is older xylem surrounding that center. Heartwood is usually much broader and is the major central support zone in a mature trunk.
You may also notice thin lines extending from the center outward in the diagram. These are rays, which move and store materials sideways across the stem. They are not cracks and are not growth rings. They help connect the radial layers, but they are not one of the six tissues you need to identify today.
A careful observation routine
If you have access to a stump, a cut firewood round, or a fallen branch with a clean cross-section, use it to practise the outside-in map. Do not remove bark from a living tree.
Work slowly:
- Find the outer bark by touch and appearance. Describe its texture, but do not infer the inner tissues from texture alone.
- Locate the pale outer wood, if visible. This is usually sapwood.
- Look toward the center for a darker zone, if present. This is often heartwood; remember that the color boundary may be faint or absent.
- Find the small central region, the pith.
- Infer rather than expect to see the vascular cambium and phloem clearly. They lie in a very thin zone between bark and sapwood, and are easiest to identify in diagrams or a very fresh cut.
A useful spoken description is:
“Outside is protective bark. Just within it is phloem, which distributes sugars. The cambium lies between phloem and wood. The outer wood is water-conducting sapwood; the inner wood is supporting heartwood; the pith is at the center.”
Being able to say this while pointing from edge to center shows that you understand both position and function.
Key takeaways
A trunk cross-section records a tree’s division of labor from the exterior inward:
- Bark is the protective covering; its outer portion is mostly dead tissue.
- Phloem, the living inner bark, distributes sugars and other organic materials between source and sink tissues.
- Vascular cambium is a very thin growth layer between phloem and xylem. It produces new phloem outward and new xylem inward.
- Sapwood is the younger outer xylem that conducts water and dissolved minerals and may store resources.
- Heartwood is older, nonconducting xylem that chiefly provides strength and often resists decay.
- Pith is the central tissue formed early in stem development, with an important storage role especially in young stems.
Next, we will leave the trunk and examine the hidden half of the tree: how root tips, root hairs, and branching root systems anchor a tree and obtain water and mineral nutrients from soil.
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