Section 13. The state may issue or guarantee the payment of bonds, provided that all bonds in excess of two million dollars shall be secured by first mortgage upon real estate in amounts not to exceed sixty-five percent of its value; or upon real and personal property of state-owned utilities, enterprises, or industries, in amounts not exceeding its value, and provided further, that the state shall not issue or guarantee bonds upon property of state-owned utilities, enterprises, or industries in excess of ten million dollars. No further indebtedness shall be incurred by the state unless evidenced by a bond issue, which shall be authorized by law for certain purposes, to be clearly defined. Every law authorizing a bond issue shall provide for levying an annual tax, or make other provision, sufficient to pay the interest semiannually, and the principal within thirty years from the date of the issue of such bonds and shall specially appropriate the proceeds of such tax, or of such other provisions to the payment of said principal and interest, and such appropriation shall not be repealed nor the tax or other provisions discontinued until such debt, both principal and interest, shall have been paid. No debt in excess of the limit named herein shall be incurred except for the purpose of repelling invasion, suppressing insurrection, defending the state in time of war or to provide for the public defense in case of threatened hostilities.