I planed a walnut board and discovered it was cut near the pith. The juvenile wood had a very serpentine path to it. You would have to cut out a 1x4 to take it out, if you wanted to. (I Didn't) From the appearance of the grain, it seems the tree tried to straighten itself. The concave areas filled in and the convex areas seemed to have less growth. It looks like the tree tried to become a straight(er) cylinder.
Do trees do that? Or am I just imagine things?