Here's some values to try :

                                r1      r2      a0      a2
                         1      0.9     0.6     pi/4    pi/4
                         2      0.9     0.9     pi/4    pi/4
                         3      0.9     0.8     pi/4    pi/4
                         4      0.9     0.7     pi/6    -pi/6


        this l-system was lifted out of the Alogorithmic Beauty of Plants.

Ed