Q13: What are L-systems?  
A13: A L-system or Lindenmayer system is a formal grammar for  
generating strings. (That is, it is a collection of rules such as replace
X with XYX.) By recursively applying the rules of the L-system to an 
initial string, a string with fractal structure can be created. Interpreting
this string as a set of graphical commands allows the fractal to be displayed.
L-systems are very useful for generating realistic plant structures.  
  
Some references are:  
  
1. P. Prusinkiewicz and J. Hanan, _Lindenmayer Systems, Fractals, and  
Plants_, Springer-Verlag, New York, 1989.  
  
2. P. Prusinkiewicz and A. Lindenmayer, _The Algorithmic Beauty of  
Plants_, Springer-Verlag, NY, 1990. ISBN 0-387-97297-8. A very good  
book on L-systems, which can be used to model plants in a very realistic  
fashion. The book contains many pictures.  
  
More information can be obtained via the WWW at:  
  
http://life.anu.edu.au/complex_systems/tutorial2.html Tutorial  
  
gopher://life.anu.edu.au:70/I9/.WWW/complex_systems/leaf.gif L-system  
leaf  
  
http://hill.lut.ac.uk:80/TestStuff/trees/ 3 Dim. L-system Tree program  
(P.J.Drinkwater)   
  
http://www.geom.umn.edu/pix/archive/subjects/L-systems.html  L-system  
images.