Here are some images I have created using my procedural model:

1. Square tree (I haven't yet figured out how to make cylinders!)

2. Branching trees (1 and 2) - created using my branching procedural model

3. Some L-Systems I have created:

F[<F]F[>F][<F[<F]F[>F]F]F[<F]F[>FF[<F]F[>F]F][<F[<F]F[>F][<F[<F]F[>F]F]F[<F]F[>F[<F]F[>F]F]F[<F]F[>F]F]F[<F]F[>F][<F[<F]F[>F]F]F[<F]F[>F[<F]F[>F][<F[<F]F[>F]F]F[<F]F[>F[<F]F[>F]F]F[<F]F[>F]F]F[<F]F[>F][<F[<F]F[>F]F]F[<F]F[>F[<F]F[>F]F]F[<F]F[>F]F

F[<F[<F]F[>F]F[<F]F[>F]F[<F]F]F[>F[>F]F[<F]F[>F]F[<F]F]F[<F[<F]<F[>F]F[<F]F]F[>F[>F]F[<F]<F]F[<F]F[>F]F

And an L-System incorporating parameters. F(x,y) => draw a line segment of width x and length y:

F(6.00,20.0)[<F(4.00,15.0)[<F(2.00,10.0)][>F(2.00,10.0)]][>F(4.00,15.0)[<F(2.00,10.0)][>F(2.00,10.0)]]