The heads of departments established in Section 2-29 of this Code shall he immediately responsible to the City Manager for the effective administration of their respective departments and all activities assigned thereto. The City Manager may set aside any action taken by such department head and may supersede him in the functions of his office. In case of a vacancy in such office or during the absence of any department head appointed by him, the City Manager may designate an interim acting head or may perform personally the functions of the office. All department heads shall keep informed as to the best practices in their particular fields and shall inaugurate, with the approval of the City Manager and subject to the limitations of the annual appropriation ordinance, such new practices as appear to be of benefit to the service and to the public.