The state treasurer shall have the power to appoint and fix the compensation of one assistant treasurer and the clerks and other employees that are necessary in the performance of his duties under the law. Each appointment, with the oath of office endorsed thereon, shall be filed in the office of the secretary of state, before the appointee enters upon his duties. The assistant treasurer shall be competent to perform and, when appointed, may perform the duties of the office. The treasurer, and his sureties on his official bond, shall be liable for the official acts, misfeasance or defalcation of the assistant treasurer.