Unless the enabling act authorizing any public body to issue bonds shall specifically exempt said bonds from the provisions of this section, or unless such enabling act shall provide a method of maturing the bonds therein authorized different from the method herein provided for, no public body shall issue its bonds, pursuant to any such general or special authority heretofore or hereafter granted, except upon a serial maturity plan, the last of such series to mature within the maximum period of maturity prescribed in the schedule of maturities hereinafter set forth dependent upon the several public purposes for which any such bonds may be issued. The public body may, in its discretion, provide for the maturity of said series in consecutive annual installments or at longer intervals, and the amounts of each series may vary, but in no event shall the final series mature after the maximum period from the date of the issue of such bonds, as hereinafter limited, shall have expired. No bonds issued for the purpose of providing funds, in whole or in part, for any of the public improvements enumerated below shall be issued to mature at a date later than the number of years from date of issue set opposite each of said public improvements so enumerated, and in no event shall bonds be issued by any public body for any public or proprietary purpose to mature more than forty (40) years from date of issue. The public improvements hereinabove referred to and the maximum number of years during which bonds for such improvements may be outstanding are as follows:

                                    Maximum Limit Upon

      Purpose of Issue      Maturity of Issue

      Bridges       30 years

      Electric light and power systems       25 years

      Gas systems       25 years

      Grade crossing eliminations       25 years

      Harbor improvements       25 years

      Highway construction       20 years

      Land acquired for permanent improvements       40 years

      Paving existing highways or streets       10 years

      School construction       25 years

      Sewerage installation       40 years

      Water systems       40 years

      Airports and building constructed or to be constructed thereon       15 years

      All other permanent structures of durable materials       25 years

      In the event any public body shall wish to provide in a single issue of bonds for two or more of the purposes hereinabove enumerated, and the maximum maturity limitations hereinabove set forth shall differ for said several purposes, then the shortest period of limitations for said several public purposes shall prevail.