Maryland’s prison agency is marking Arbor Day by putting inmates to work planting trees. The Department of Public Safety and Correctional Services said it is approaching the halfway point in its effort to plant a million trees across the state by 2012. The agency says inmates will plant nearly 400,000 seedlings this spring in Cecil, Frederick, Montgomery, Somerset, Washington and Baltimore counties. That will bring to more than 600,000 the number planted since Arbor Day of 2008. The effort is a partnership between the prison agency and the state departments of natural resources and transportation.