When the governing body of any city shall deem it necessary in order to pay for lands hereafter or heretofore appropriated or purchased for opening, widening, or extending any street or alley it may issue and sell to obtain funds therefor the negotiable bonds of such city in a sum not to exceed the cost thereof. Said board shall levy and certify to the county clerk, to be placed upon the tax rolls, a tax sufficient to pay the interest on said bonds and the principal at maturity.