Any coupons for the payment of interest on metropolitan park district bonds shall be considered for all purposes as warrants drawn upon the metropolitan park district fund against which the bonds were issued, and when presented after maturity to the treasurer of the county having custody of the fund. If there are no funds in the treasury to pay the coupons, the county treasurer shall endorse said coupons as presented for payment, in the same manner as county warrants are endorsed, and thereafter the coupon shall bear interest at the same rate as the bond to which it was attached. If there are no funds in the treasury to make payment on a bond not having coupons, the interest payment shall continue bearing interest at the bond rate until it is paid, unless otherwise provided in the proceedings authorizing the sale of the bonds.