No purchase, construction, alteration or repair shall be made except upon a requisition by the director or the officer at the head of the department for which the same is to be made or done or upon the order of Council. However, nothing in this section shall be construed to mean that the Division of Purchase, Construction and Repair has no right to purchase supplies to be kept in stock and furnished to departments on requisition. Such requisition shall be accompanied by a sample of the material or supplies to be purchased or shall contain specifications or plans, or both, adequate to describe the requirements set forth in the requisition.
   The director or head of each department shall sign or approve with his signature all requisitions of his department. In departments where there are subdepartments or divisions, each requisition shall be signed by the chief of such subdepartment or division and shall be approved by the director or officer at the head of such department. There shall be attached to each requisition a statement setting forth the necessity for the articles requisitioned.
(1969 Code Sec. 31.42)