You're probably sick of my hierarchy posts, so here're some great paintings by Mel Crawford (which incidentally are full of hierarchical thinking)
Check out the clever texture of the tree. All the lines are asymmetrical-no parallels - but they all flow along the shape of the tree. Note also, the spacing of the bark lines: They aren't evenly spaced; some are close together in groups and separated from other groups.
The construction and sub-forms of this tree house are wonderful!
Thanks to Barbie and her great site: