Section 68. The terms of office of the members of the council shall commence at ten o’clock in the forenoon of the first Monday in January following their election and shall be for two years each, and until their successors are qualified; provided, that, if the plan adopted provides for elections to be held annually, the terms of office of the commissioner of administration, of the commissioner of finance and of the commissioner of health elected at the first regular election held under such plan shall be for two years, and the terms of office of the commissioner of public works and of the commissioner of public property so elected shall be for one year.