I think the usual idea is that during development, cell division patterns in the plant apical meristem [1] implement some Lindenmayer system [2]. I don't know if anyone has looked into genetic & molecular mechanisms whereby cell division could be regulated so as to implement an L-system in this way.
[1] http://en.wikipedia.org/wiki/Meristem
[2] http://en.wikipedia.org/wiki/L-system
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edit: lots of stuff here: http://algorithmicbotany.org/papers/