In case you can't see, that's a tree with its roots on the foot and branches reaching around the calf. I'd been trying to think of a way to pull this off on an arm, but then I realized that the contours of the leg are much more appropriate. The final version would be much better, of course, possibly with leaves and blooms and such. Though this version came out better than I thought it would.
(And yes I do realize that tattooing on the foot would be considerably more painful and less likely to come out picture-perfect. I can live with that.)