Abstract: In this paper, we present an FL-system, an extension of an L-system that allows us to generate any kind of object hierarchy and mesh on the fly. This has been made possible thanks to a modification of the classical L-system rewriting mechanism that produces a string of symbols interpreted afterwards. In our system, terminal symbols are not characters, but functions that can be executed at any step of the rewriting process. Thanks to this extension, our system allows the instantiation of generic objects during the course of the rewriting process as well as their initialization. Therefore, we are able to simulate all of the existing solutions proposed by classical L-systems, but we are also able to generate VRML97 scene graphs and geometry on the fly, since VRML97 nodes are handled as generic objects. As an example, we will show in the second part of this paper how to use our extension to describe building styles that are utilized to generate large sets of different building models. We also present some models of urban features (street lamps, etc.) and plants modeled and generated using FL-systems.