Mo. Rev. Stat. § 21.090

If any member elected to either house of the general assembly resigns in the recess thereof, he shall address and transmit his resignation, in writing, to the governor; and when any member resigns during any session, he shall address his resignation, in writing, to the presiding officer of the house of which he is a member, which shall be entered on the journal; in which case, and in all cases of vacancies happening, or being declared, during any session of the general assembly, by death, expulsion or otherwise, the presiding officer of the house in which the vacancy happens shall immediately notify the governor thereof.