cambium

Medieval Latin cambium meant exchange, from the verb cambīre (to trade, to barter)—the same root that gives us change and exchange. Botanists adopted the word in the seventeenth century for the thin layer of actively dividing cells between a tree's bark and its wood. The cambium is where exchange happens: nutrients from the soil travel up through the sapwood; sugars from the leaves travel down through the bark. The cambium sits at the intersection, building new cells in both directions.

Every tree ring is a product of the cambium. In spring, the cambium produces large, thin-walled cells (earlywood). In summer and fall, it produces smaller, thick-walled cells (latewood). The alternation creates the visible rings that dendrochronologists read to determine a tree's age and the climate conditions of each year. The cambium is the tree's pen, writing one ring per year in a language scientists learned to read.

The cambium is extraordinarily thin—often just a few cells thick. Yet it is the only part of a mature tree that is actively growing. The heartwood at the center is dead. The bark on the outside is dead or dying. The cambium is the living margin between two kinds of death, the narrow band where the tree is still becoming.

Grafting, the technique that produces most commercial fruit trees, works because the cambium of the scion must align with the cambium of the rootstock. If the two cambium layers do not make contact, no vascular connection forms, and the graft fails. The success of an orchard depends on a thin line of cells—the exchange layer where two trees agree to become one.