Abstract: Row marker assemblies are pivotally mounted near the ends of a tool bar adapted to carry various types of implements. A pair of marker control arms near the center of the tool bar are connected with the two marker assemblies for controlling the raising and lowering of the latter. A single hydraulic cylinder interconnects the control arms and regulates their movements in cooperation with gravity-responsive pivoted latch bars on the control arms. The latch bars are alternately engaged with a stationary keeper attached to the tool bar and are alternately separated therefrom by a camming and lifting action of each latch bar in response to retraction of the single cylinder, the lifting of each latch bar serving to release the opposing latch bar from the keeper, and the first latch bar then moving by gravity into positive locking engagement with the keeper. Rotational elements, springs and other external controls on the two cooperating latch bars are eliminated.