Here is an applet to try out Lindenmayer's language. Type in the parameters and watch the result after pressing return or clicking the update-button. If you don't know my version of his language, use the change figure-button to view the examples first.

Look at the symbols list and get more information about the L-system.
 

In the year 1968 the german biologist Aristid Lindenmayer developed a model, demonstrating plant's growth with the use of some "construction rules". Today we name it Lindenmayer System or shorter L-System. 

The basic principle is simple: Starting with the axiom as the first element, the construction rules form complex expressions which have to be drawn. We only need line elements of a fixed basic length (F, G). In addition there are rotations by a standard angle.

Our growing machine is controlled by up to two construction rules. These rules induce  the exchange of single symbols by longer expressions in order to form more complex constructions  of the same kind. The new expression contains the same symbols, so the following step has to replace them again. The number of steps is called the order.

The final expression forms the drawing with line elements, rotations and unvisible jumps. An example will show the mechanism:

axiom: F
rule: F:F+F--F+F

order:
0: F
1: F+F--F+F
2: F+F--F+F+F+F--F+F--F+F--F+F+F+F--F+F
3: ...
Every F is replaced by F+F--F+F, the other symbols remain unchanged.
Important: The rule is formed of symbol, partition sign and replacement expression