All of the bonds shall be executed and delivered to the secretary-treasurer of the district, who shall sell the bonds in the quantities and at the dates which the board of commissioners may deem necessary to meet the payments for the works and improvements in the district.  The bonds shall not be sold for less than ninety per cent of par and accrued interest to date of delivery, and shall show on their face the purpose for which they are issued, and shall be payable out of money derived from the taxes levied as provided by previous sections of this Part.