Mo. Const. art. III, § 29

No law passed by the general assembly, except an appropriation act, shall take effect until ninety days after the adjournment of the session in either odd-numbered or even-numbered years at which it was enacted. However, in case of an emergency which must be expressed in the preamble or in the body of the act, the general assembly by a two-thirds vote of the members elected to each house, taken by yeas and nays may otherwise direct; and further except that, if the general assembly recesses for thirty days or more it may prescribe by joint resolution that laws previously passed and not effective shall take effect ninety days from the beginning of the recess. [Effective - 27 Feb 1945 , see footnote]