Cell-wall modifications in the root tissues surrounding the lateral emerging roots of leek were investigated using morphological and in situ techniques. Primordium meristem cell walls were thin with a weak gold labelling after treatments for pectin and cellulose localization, as already described in other plant meristems. By contrast, the surrounding tissues of the mother roots displayed deep changes: their walls were swollen, thickened, with large intercellular spaces. Specific probes revealed a distribution of cellulose molecules comparable to the undisturbed root and a substantial increase in pectic material. Cell-wall remnants were at the interface between the emerging roots and the mother root. They were rich in pectic material, but not in cellulose. Other immunogold experiments located a polygalacturonase over the primordium cells and its interface with the mother root. It is suggested that lateral root morphogenesis involves controlled cell separation, thanks to a specific interaction between pectinolytic enzymes and pectins.