Mo. Rev. Stat. § 168.271

The first year of employment of all employees covered by sections 168.251 to 168.291 * shall be deemed a period of probation. During the probationary period, any probationary employee whose work is found to be unsatisfactory shall be furnished a written statement by the administrative officer of the department in which he is employed, setting forth the nature of his unsatisfactory work or incompetency and a copy of the statement shall be filed with the personnel director. If improvement satisfactory to the administrative officer of the department is not made within one month after the receipt of the statement, the probationary employee shall be discharged.