All certificates of indebtedness of the state to the public school fund and to the seminary fund are hereby confirmed as sacred obligations of the state to said funds, and they shall be renewed as they mature for such time and at such rate of interest as may be provided by law. The general assembly may provide at any time for the liquidation of said certificates, but all funds derived from such liquidation, and all other funds hereafter accruing to said state school or state seminary funds, except the interest on same, shall be invested only in registered bonds of the United States or the state, bonds of school districts of the state, or bonds or other securities payment of which are fully guaranteed by the United States, of not less than par value. The general assembly may levy an annual tax sufficient to pay the accruing interest of all state certificates of indebtedness.