The directors are empowered, whenever they deem it advisable, to invest any sinking fund of the district in bonds of the United States, of this State, of any county of Texas, any irrigation or water improvement or navigation bonds, or bonds of any school district in Texas authorized to issue bonds. No bonds shall be so purchased whose terms provide for their maturity at a date subsequent to the time of the maturity of the bonds for the payment of which such sinking fund was created.