Is there a mathematical theory that explains the shape of a snowflake? Why is it not round?

<b> Update</b> Tree-like structures appear often as limits of sequences of metric spaces (say, <a href="http://front.math.ucdavis.edu/0405.5030">asymptotic cones</a> or <a href="http://www.warwick.ac.uk/~masgak/abstracts/qid.html"> boundaries</a> of metric spaces). I wonder if similar structures can be obtained as shapes minimizing some kind of energy functional. This may lead to new constructions in geometric group theory. 

I just saw Igor Rivin's answer which may be what is needed. Perhaps somebody can give a more detailed answer?