INADEQUATE PREFLIGHT BY THE STUDENT PILOT, INADEQUATE SUPERVISION BY THE FLIGHT INSTRUCTOR (CFI), AND FUEL EXHAUSTION DUE TO LACK OF FUEL. RELATED FACTORS WERE: INOPERATIVE LEFT FUEL QUANTITY GAGE, THE CFI'S DECISION TO MAKE THE FLIGHT WITH A KNOWN DEFICIENCY IN THE FUEL GAGE, AND OBSTRUCTIONS (TREE LIKE CACTUS) IN THE EMERGENCY LANDING AREA.