Final answer:
Gravitropism is the term for a plant's roots growing downwards in response to gravity, which is known as positive gravitropism. Special cells in the root caps detect gravity and use amyloplasts to direct growth appropriately.
Step-by-step explanation:
The roots of a plant growing down in response to gravity is called gravitropism. This phenomenon ensures that roots grow into the soil, which is called positive gravitropism, while the shoots grow upwards, known as negative gravitropism.
Plants respond to gravity through specialized cells found in the root caps, which can detect gravity and stimulate growth in the direction of the Earth's gravitational pull. Amyloplasts in these cells, which are dense and fall in the direction of gravity, play a key role in this process.