Public debt other than certificates of indebtedness authorized in section 6 shall be evidenced by the issuance of bonds of the state. All bonds issued under the provisions of this section shall mature not more than 20 years from their respective dates of issue and each law authorizing the issuance of bonds shall distinctly specify the purposes thereof and the maximum amount of the proceeds authorized to be expended for each purpose. A separate and special state bond fund shall be maintained on the official books and records. When the full faith and credit of the state has been pledged for the payment of bonds, the state auditor shall levy each year on all taxable property within the state a tax sufficient with the balance then on hand in the fund to pay all principal and interest on bonds issued under this section due and to become due within the ensuing year and to and including July 1 in the second ensuing year. The legislature by law may appropriate funds from any source to the state bond fund. The amount of money actually received and on hand pursuant to appropriations prior to the levy of the tax in any year shall be used to reduce the amount of tax otherwise required to be levied.
