How does a skinny sapling become a giant tree trunk? Long, thin cells hidden under the bark keep making new wood, year after year.
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In woody plants, a thin layer of dividing cells wraps all the way around the stem, just under the bark. It is called the vascular cambium.
Most of its cells are fusiform initials. "Fusiform" means spindle-shaped: long and thin with pointy ends. They are many times longer than they are wide, and they stand up and down along the trunk.
Fusiform initials divide along their length. The new cells go two ways:
They usually make much more wood than phloem. That is why a tree trunk is mostly wood!
Press Make wood and then Make phloem on the 3D model to see new cells appear on each side.
In places with seasons, the cambium makes a new band of wood each year. Those bands are the tree rings you can count on a stump!