1. A GUIDANCE SYSTEM FOR AUTOMATICALLY CONTROLLING HYDROSTATIC POWER STEERING APPARATUS OF A TRACTOR HAVING A RECIPROCATING HYDRAULIC MOTOR THEREIN AND HAVING AT LEAST ONE FRONT WHEEL ROTATIVE ABOUT A HORIZONTAL AXLE, SAID AXLE BEING PIVOTAL ABOUT A VERTICAL AXIS RESPONSIVE TO THE OPERATION OF THE RECIPROCATING HYDRAULIC MOTOR OF SAID APPARATUS, SAID APPARATUS INCLUDING HYDRAULIC CIRCUITRY AND A VALVE IN SAID CIRCUITRY HAVING A SPOOL OPERATIVELY CONNECTED WITH THE PISTON OF A FLUID PRESSURE MOTOR FOR CONTROLLING THE OPERATION OF THE RECIPROCATING HYDRAULIC MOTOR OF SAID APPARATUS, SAID SYSTEM COMPRISNG A HYDRO-MECHANICAL UNIT CARRIED BY SAID TRACTOR FOR PIVOTAL MOVEMENT WITH SAID AXLE ABOUT SAID AXIS, SAID UNIT INCLUDING (A) A HOUSING DISPOSED FORWARDLY OF SAID TRACTOR, (B) A DEPENDING SHAFT SUPPORTED BY SAID HOUSING FOR RELATIVE ROTATION ABOUT A VERTICAL AXIS, (C) A GUIDE CASTER SUPPORTED BY SAID SHAFT IN PREDETERMINED SPACED RELATION WITH SAID SHAFT AND SAID WHEEL, SAID CASTER BEING ROTATIVE ABOUT ONE HORIZONTAL AXIS AND PIVOTAL ABOUT ANOTHER HORIZONTAL AXIS IN PREDTERMINED SPACED RELATION WITH SAID ONE HORIZONTAL AXIS, SAID CASTER ALSO BEING PIVOTAL ABOUT SAID VERTICAL AXIS OF SAID SHAFT, (D) A CAM MOUNTED ON SAID SHAFT FOR ROTATION THEREWITH, (E) AN ACTUATOR VALVE FOR CONTROLLING ACTUATION OF SAID PISTON AND INCLUDING A SPOOL SPRING-BIASED IN NEUTRAL POSITION, (F) SAID ACTUATOR VALVE HAVING PRESSURE FLUID INLET AND OUTLET PORTS IN OPERATIVE COMMUNICATION WITH CORRESPONDING PORTS OF SAID FLUID PRESSURE MOTOR, (G) LINKAGE MECHANISM OPERATIVELY INTERCONNECTING SAID CAM AND SAID ACTUATOR VALVE SPOOL WHEREBY UPON PREDETERMINED MOVEMENT OF SAID CAM IN ONE DIRECTION, AND ANOTHER, SAID ACTUATOR VALVE SPOOL MOVES CORRESPONDINGLY TO OPEN AND CLOSE THE PORTS OF SAID ACTUATOR VALVE, RESPECTIVELY, AND THUS TO OPERATE THE SAME AND TO ACTUATE SAID FLUID PRESSURE MOTOR THEREBY TO ACTUATE SAID FIRST VALVE AND CAUSE THE RECIPROCATING HYDRAULIC MOTOR OF SAID STEERING APPARATUS TO BE OPERATED CORRESPONDINGLY, (H) SAID CASTER BEING ARRANGED TO TRAVEL IN A GROUND MARKER GROOVE INDICATING A SELECTED PATH OF TRAVEL FOR SAID TRACTOR, AND SAID CAM BEING PIVOTALLY RESPONSIVE TO CHANGES IN DIRECTION OF TRAVEL OF SAID CASTER.