One solution is to define the symbol F
in a way which reduces the value of \pgflsystemstep
, i.e:
\pgfdeclarelindenmayersystem{A}{
\symbol{F}{\pgflsystemstep=0.6\pgflsystemstep\pgflsystemdrawforward}
\rule{A->F[+A][-A]}
}
Which gives:

Complete code for the above figure:
\documentclass{article}
\usepackage{tikz}
\usepackage[active,tightpage]{preview}\PreviewEnvironment{tikzpicture}
\usetikzlibrary{lindenmayersystems}
\pgfdeclarelindenmayersystem{A}{
\symbol{F}{\pgflsystemstep=0.6\pgflsystemstep\pgflsystemdrawforward}
\rule{A->F[+A][-A]}
}
\begin{document}
\foreach \n in {1,...,8} {
\begin{tikzpicture}[scale=10,rotate=90]
\draw (-.1,-.2) rectangle (.4,0.2);
\draw
[blue,opacity=0.5,line width=0.1cm,line cap=round]
l-system [l-system={A,axiom=A
,order=\n,angle=45,step=0.25cm}];
\end{tikzpicture}
}
\end{document}
Update
A better solution: Define a new symbol (eg, S
for "scale") which changes the scale, but draws nothing. The advantage is that it can be used in any part of the rule.
Using this approach, your example would be:
\pgfdeclarelindenmayersystem{A}{
\symbol{S}{\pgflsystemstep=0.6\pgflsystemstep}
\rule{A->SF[+A][-A]}
}
Another example, which shows how the "scaling" can be used in a part of a rule, could be:
\pgfdeclarelindenmayersystem{A}{
\symbol{S}{\pgflsystemstep=0.5\pgflsystemstep}
\rule{A->F[+A][-A]SA}
}
which, used this way:
\foreach \n in {1,...,6} {
\begin{tikzpicture}[scale=10,rotate=90]
\draw (-.03,-.17) rectangle (.35,0.17);
\draw
[blue,opacity=0.5,thin,line cap=round]
l-system [l-system={A,axiom=A
,order=\n,angle=15,step=0.05cm}];
\end{tikzpicture}
}
produces
