Part of an educational set for exploring wood structure for timber grading.
This is a simplfied model of a sawn log showing growth rings, sapwood and heartwood, and knots on the outside surface.
When the log is sawn, the grain, knots and wane depend on the direction of the cut. The wood is shown with transparency to reveal the knots and pith inside.
https://blogs.napier.ac.uk/cwst/3dwood/