06 January 2015
This image was originally posted as just a "tree within a tree" curiosity, but it set me off on a search to understand why the heartwood of a tree would take on such an asymmetric pattern.
I found some images at treedictionary.com that are useful, in that they show trees sectional sagitally rather than transversely.
Apparently the transverse asymmetry arises when the heartwood branches out into one of the limbs of the tree. Some readers here may be able to clarify or fill in more details from their own experience or work.
Cropped for size from the original at Reddit.