The Council shall establish a purchasing agency either as a separate office or as a division of one of the departments created by this Charter, whose duty shall be to. contract for, purchase, store, and distribute all supplies, printing, materials, and equipment required by any agency of the City government, within the appropriations made by the Council and allotments made by the City Manager pursuant to this Charter. The Council may designate the City Manager as purchasing agent but in any case the City Manager shall be responsible for supervising the purchasing function.