Mo. Rev. Stat. § 355.341: Resignation of directors.

1. A director may resign at any time by delivering written notice to the board of directors, its presiding officer or to the president or secretary.

2. Unless otherwise stated in the bylaws or articles of incorporation, a resignation is effective when the notice is delivered unless the notice specifies a later effective date. If a resignation is made effective at a later date, the board may fill the pending vacancy before the effective date if the board provides that the successor does not take office until the effective date.