The positive gravitropism is the movement or response of the plants towards the force of gravity. The growth of root is an example for the positive gravitropism. The primary roots grow towards the force of gravity that is in the downward direction.
Whereas the negative gravitropism is the movement or response of the plant against the force of gravity. The growth of shoot of the plant is an example of negative gravitropism. Here the shoot grows away from the earth that is in the upward direction.