I've modified it slightly. To me the most impressive thing about it is that you can redefine one of the functions, without closing the window displaying the tree, and then as soon as you resize the window the tree is redrawn using the new definition.
This makes the process of trying out new formulae great fun.
(import '(javax.swing JFrame JPanel ) '(java.awt Color Graphics Graphics2D)) (defn draw-tree [g2d angle x y length branch-angle depth] (if (> depth 0) (let [new-x (+ x (* -1 length (Math/sin (Math/toRadians angle)))) new-y (+ y (* -1 length (Math/cos (Math/toRadians angle)))) new-length (fn  (* length (+ 0.75 (rand 0.1)))) new-angle (fn [op] (op angle (* branch-angle (+ 0.75 (rand)))))] (. g2d drawLine x y new-x new-y) (draw-tree g2d (new-angle +) new-x new-y (new-length) branch-angle (- depth 1)) (draw-tree g2d (new-angle -) new-x new-y (new-length) branch-angle (- depth 1))))) (defn render [g w h ] (doto g (.setColor (Color/BLACK)) (.fillRect 0 0 w h) (.setColor (Color/GREEN))) (let [init-length ( / (min w h) 5), branch-angle (* 10 (/ w h)), max-depth 10] (draw-tree g 0.0 (/ w 2) h init-length branch-angle max-depth))) (defn create-panel  "Create a panel with a customised render" (proxy [JPanel]  (paintComponent [g] (proxy-super paintComponent g) (render g (. this getWidth) (. this getHeight))))) (defn run  (let [frame (JFrame. "Fractal Tree") panel (create-panel)] (doto frame (.add panel) (.setSize 640 400) (.setVisible true)))) (run)