In case any temporary or unforeseen exigency should arise in any of the county offices, departments or institutions and it shall be made to appear upon the written statement of the county officer, or head of the county department or institution that a serious detriment will be caused to the service if the method of purchase as hereinbefore defined shall be pursued, then such officer or head, if upon examination he shall deem an immediate purchase necessary, may by appropriate order to be approved by the purchasing agent and duly entered upon his records, authorize the purchase of such supplies as may be needed to meet such temporary and unforeseen exigency, and which are not embraced in any existing contract, and the county officer or head of a county department or institution may thereupon purchase the same in open market. A report of such purchase, together with a copy of the application of the officer or head, shall be transmitted by the purchasing agent, with his endorsement thereon, to the county auditors, and upon it the county auditors shall issue their warrant upon the treasury of the county for the amount so expended. The provisions of this section shall only apply to articles and supplies that are not of a strictly perishable character.