Cells lining a tunnel
Pines, spruces, and many of their relatives have tiny tunnels running through their wood, bark, and needles. These tunnels are called resin canals.
Each canal is lined with a ring of small, living epithelial cells. They are packed close together around the open middle of the tunnel, like bricks lining a well.
Try this
Find a pine tree and look at its bark (look, don't touch!). You may see shiny, sticky drops or white crusty spots. That's resin! Crush a pine needle between your fingers and smell the piney scent.