Gravitropism is when the stem of a plant grows upwards and the root of the plant grows downwards. This phenomenon was discovered by Darwin back in the days and basically implies that roots always grow downwards in the direction of the gravity force and the stem always grows upwards, in the opposite direction of the gravity force. You can easily check this by putting a potted plant on a side. You will observe the stem bend upwards against the gravity force and the roots will bend downwards in the direction of the gravity force.
The tree above is a gravitropic mutant, with the root and the stem swapping growth directions. This is probably the only known tree in the world that grows downwards. It grows in Italy in the remains of a cellar.