According to http://www.house.mo.gov/content.aspx?info=/info/howbill.htm

VII. SIGNING BY THE GOVERNOR

Bills Truly Agreed To and Finally Passed are signed in open session by the Speaker of the House and the President Pro Tem of the Senate. At the time of signing, any members may file written objections which are sent with the bill to the Governor. The Governor has fifteen days to act on a bill if it is sent to him during the legislative session; and forty-five days if the legislature has adjourned or has recessed for a thirty day period. The Governor has four options:

1. Sign the bill, making it become part of Missouri law.

2. Veto the bill. In this case, the bill is returned to the General Assembly where a two-thirds vote of both houses is required to override the veto.

3. Not sign the bill. Should the Governor take no action within the prescribed time, the bill goes to the Secretary of State, who then enrolls the bill as an authentic act. It then becomes law.

4. Veto line-items in an appropriation bill. On appropriation bills only, the Governor may choose to veto selected items within the bill. The General Assembly may override this veto by a two-thirds majority of both houses.

-Jonathan