"SEC. 79 (C). Power of direction and supervision.—The Department Head shall have direct control, direction, and supervision over all bureaus and offices under his jurisdiction and may any provision of existing law to the contrary notwithstanding, repeal or modify the decisions of the chiefs of said bureaus or offices when advisable in the public interest.
"The Department Head may order the investigation of any act or conduct of any person in the service of any bureau or office under his department and in connection therewith may appoint a committee or designate an official or person who shall conduct such investigations, and such committee, official, or person may summon witnesses by subpoena and subpoena duces tecum, administer oath and take testimony relevant to the investigation."