Abstract: To permit selective operation of an application roller either transferring wetting water and ink, or only wetting water, depending on subject matter to be printed on an offset printing machine, without disturbance of the ink distribution system thereof, the application roller is movably positioned to be, selectively, in engagement with an ink distribution roller or out of engagement therewith, the application roller being maintained in continuous engagement with the plate cylinder and a wetting liquid transfer roller being in continuous engagement with the application roller. Preferably, and to permit movement, engagement which includes the application roller is by a resilient bias force, for example supplied by a spring moving the water transfer roller towards the application roller, or the application roller itself against the plate cylinder.