particular row of pot cells which row was memorized as to its absence of acceptable seedlings at particular locations becomes aligned during movement with plunger row pr the tray is momentarily stopped and selected plungers are operated to empty the remembered pot cells to prepare them to receive fresh replacement seedlings from say another tray so that the occupancy of the cells in the tray being inspected can be restored to be full each time the plungers pr act to evacuate selected pot cells the plungers are retracted to their rest position so that the trays containing seedlings can be moved forward by a predetermined number of notches in the event however any of the plungers fails to retract owing to a malfunction the situation is diagnosed and the operation is temporarily interrupted manual intervention is called for so as to restore the normalcy of the apparatus if the trays are advanced when the plungers have failed to retract some of the seedlings are likely to be malled and such a situation is undesirable in order to detect a malfunction of one or more plungers the arrangement as illustrated uses laser beams ls which are aligned to pass through each column of plungers and impinge on photodiodes pd any plunger which stays in a lowered position and fails to retract because of a malfunction is immediately diagnosed by the corresponding laser being interrupted by the malfunctioning plunger whereupon the recipient photodiode by virtue of the absence of an incident laser.