Final answer:
Auxins flow downwards due to gravitropism, a response to the direction of gravity. The higher concentration of auxin on the lower side of the root inhibits cell elongation, causing the root to grow downward.
Step-by-step explanation:
Auxins are plant hormones that control cell elongation in response to gravity. This downward flow of auxin is called gravitropism, a type of tropism where plants respond to the direction of gravity. When a plant is tilted, specialized cells in the root cap detect the change in orientation and trigger the movement of auxin towards the lower side of the root. The higher concentration of auxin on the lower side inhibits cell elongation, causing the root to grow downward.