The style of the laws of this state shall be: "Be it enacted by the legislature of Alabama," which need not be repeated, but the act shall be divided into sections for convenience, according to substance, and the sections designated merely by figures. Each law shall contain but one subject, which shall be clearly expressed in its title, except general appropriation bills, general revenue bills, and bills adopting a code, digest, or revision of statutes; and no law shall be revived, amended, or the provisions thereof extended or conferred, by reference to its title only; but so much thereof as is revived, amended, extended, or conferred, shall be re-enacted and published at length.