1. Bonds issued pursuant to sections 253.210 to 253.280 shall be of such denomination, shall bear such rate of interest, not to exceed fourteen percent per annum, and shall mature at such time, within forty years from the date of issuance, as the department of natural resources may determine. The bonds may be either serial or term bonds.

2. Serial bonds may be issued with or without the reservation of the right to call them for payment and redemption in advance of their maturity, upon giving such notice, and with or without a covenant requiring the payment of a premium in the event of payment and redemption prior to maturity as the department of natural resources may determine.

3. Term bonds shall contain a reservation of the right to call them for payment and redemption prior to maturity at such time and upon the giving of such notice and upon the payment of such premium, if any, as the department of natural resources may determine.

4. The bonds, when issued, shall be sold at public sale for the best price obtainable after giving such reasonable notice of the sale as the department of natural resources may determine except that no bonds shall be sold for less than ninety-five percent of their par value, and accrued interest.

5. The bonds may be sold to the United States of America or to any of its agencies or instrumentalities, at a price not less than par and accrued interest, without public sale and without the giving of the notice prescribed in this section.

6. The bonds, when issued and sold, shall be negotiable instruments within the meaning of the law merchant and the negotiable instruments law, and the interest thereon shall be exempt from income taxes under the laws of this state.