Stochastic L-Systems

This is a first attempt to generate Stochastic L-systems as described in The Algorithmic Beauty of Plants (pp28-30).  I've yet to produce any convincing plant-like forms, but it's a start.  
Note: I haven't tried too hard to break this, and I'm just getting into Python coding.  Please let me know how and when it fails.  In the off-chance it does something cool for you, please let me know.