Secondary xylem and phloem cells arise from the vascular cambium, a thin cylinder of dividing cells that forms a continuous ring inside the stems and roots of woody plants and many herbaceous dicots. This layer of tissue sits between the existing xylem and phloem, and it produces new cells in both directions: secondary xylem toward the center of the stem and secondary phloem toward the outside.
What the Vascular Cambium Is
The vascular cambium is a lateral meristem, meaning it’s a zone of actively dividing stem cells that runs lengthwise through a plant’s stems and roots. Unlike the meristems at shoot and root tips that make a plant taller, the vascular cambium makes a plant wider. It’s responsible for what botanists call secondary growth, the thickening you see in tree trunks, woody shrubs, and even the roots of plants like cassava.
The cambium itself originates from two sources. Between the vascular bundles, it develops from procambial cells, which are embryonic vascular tissue embedded between the primary xylem and phloem. In the regions between bundles (in shoots), it arises from ordinary parenchyma and endodermis cells that resume dividing. These two sources eventually merge into a single continuous ring. In roots, this ring forms as the pericycle cells (the outermost layer of the central vascular cylinder) begin dividing alongside the procambium, often as early as five days after germination in fast-growing species.
How the Cambium Produces New Cells
The vascular cambium contains a narrow central zone of bifacial stem cells, meaning each initial cell can generate daughters in two directions. When a cambial initial divides periclinally (the new cell wall forms parallel to the stem surface), one daughter remains as a stem cell while the other is pushed either inward or outward. Daughters pushed inward become xylem mother cells, which divide further and mature into secondary xylem. Daughters pushed outward become phloem mother cells, differentiating into secondary phloem.
This inward-outward pattern is why trees accumulate far more wood than bark. The cambium typically produces more cells on the xylem side, and as that wood accumulates, it physically pushes the cambium outward, increasing its circumference. To keep up, the cambial initials also divide anticlinally (with the new wall oriented perpendicular to the surface). Each anticlinal division adds one new radial file of cells, letting the cambium expand its ring to match the growing trunk diameter. Notably, anticlinal divisions are only necessary when xylem production is pushing the cambium outward. When the cambium is producing phloem, its circumference isn’t affected, so no anticlinal divisions are needed.
Two Types of Cambial Initials
The cambium contains two distinct kinds of stem cells, each responsible for different structural elements. Fusiform initials are long, vertically oriented cells that produce the axial (lengthwise) components of both secondary xylem and phloem. Ray initials are smaller, roughly cube-shaped cells that produce rays, the horizontal ribbons of mostly parenchyma tissue that run from the center of the stem outward like the spokes of a wheel. Rays serve as pathways for moving water, sugars, and other materials laterally across the trunk.
What Secondary Xylem Contains
Secondary xylem, commonly known as wood, is made up of four main cell types. Tracheids and vessel elements are the water-conducting cells; they die at maturity and form hollow tubes reinforced with lignin. Tracheids are the sole conducting cells in conifers and other gymnosperms, while flowering plants have both tracheids and the wider, more efficient vessel elements. Fibers provide mechanical strength. Parenchyma cells remain alive and handle storage and lateral transport.
As a tree ages, the oldest secondary xylem near the center of the trunk undergoes a transformation. The living parenchyma and ray cells die through programmed cell death, starch and sugars are depleted, and chemical compounds called extractives accumulate in the cell walls. This often darkens the wood, producing what’s called heartwood. Heartwood no longer conducts water or stores nutrients, but it continues to support the tree structurally. The outer, younger secondary xylem that still conducts water is called sapwood.
What Secondary Phloem Contains
Secondary phloem is the tissue responsible for transporting sugars, amino acids, hormones, and signaling molecules from leaves to the rest of the plant. Its key functional unit is the pairing of sieve elements and companion cells. Sieve elements are highly specialized conduits that break down their own nucleus and most organelles during development, creating an open channel for sap flow. Because they lack a nucleus, they depend on their neighboring companion cells to stay alive. The two cell types are connected by dense clusters of plasmodesmata (tiny channels through the cell wall) that allow the companion cell to supply the sieve element with proteins, energy, and regulatory molecules.
Secondary phloem also contains fibers for structural support and parenchyma cells for storage. Unlike secondary xylem, which accumulates year after year as wood, older secondary phloem gets crushed and compressed as the trunk expands. Only a thin layer of functional phloem is maintained at any given time.
What Controls Cambial Activity
The hormone auxin is the primary driver of cambial cell division. Synthesized in the shoot tip, auxin travels downward through the stem via specialized transport proteins and maintains the cambium’s stem cell identity. It acts through a set of transcription factors that define where the cambium sits and keep it actively dividing.
Auxin doesn’t work alone. Cytokinin plays a major role in defining whether a cambial daughter becomes a xylem or phloem cell and in controlling the rate of cell division. Gibberellin promotes cell elongation and triggers the deposition of the thick, lignified secondary cell walls that characterize mature xylem. The interplay among these three hormones is complex: in cassava roots, for example, a strong auxin signal coincides with cambium formation, while gibberellin signaling actually decreases during the transition to secondary growth.
Seasonal Patterns in Secondary Growth
In temperate climates, the vascular cambium is not active year-round. It resumes dividing in spring as temperatures rise and water becomes available, producing large, thin-walled xylem cells (called earlywood) that maximize water transport. As summer progresses and conditions become drier, the cambium shifts to producing smaller, thicker-walled cells (latewood) that are mechanically stronger. This seasonal alternation creates the visible annual rings in a cross-section of wood.
Environmental variables like temperature, day length, and water availability all influence how many cells the cambium produces and when it goes dormant for winter. Photosynthetic activity in the leaves is closely tied to xylem formation, since the sugars produced by photosynthesis fuel the energy-intensive process of building new cell walls and depositing lignin. Studies on tree species with different growth habits show that traits like vessel density and transpiration rate shift substantially across the growing season, reflecting the tight coupling between leaf activity and cambial output.