Polysilicon crystalline rods are formed by chemical vapor deposition in the reaction chamber of a Siemens reactor. Filament holding assemblies secure vertically extending filaments to electrodes located along the floor of the reactor. A filament holding assembly includes a chuck support member that is mounted on an electrode and that has an upwardly tapering side surface. A chuck is seated on the chuck support member with at least a portion of the chuck support member received within a cavity defined in the base of the chuck with the side surface of the chuck support member engaging the surface that defines the cavity. The cavity can sized and shaped such that a gap is defined between the distal end of the chuck support member and an end wall surface of the cavity. The chuck has an upwardly opening receptacle that receives and holds the end portion of an upwardly extending filament.