From Seed to Seed:
Plant Science for K-8 Educators

Response to Gravity

Have you ever wondered why roots grow down and shoots grow up-no matter how you plant a seed? The term gravitropism describes how plants respond to gravity. Roots are termed positively gravitropic; that is, they grow toward the direction of the pull of gravity. Shoots are negatively gravitropic because they grow away from that force. If you plant a seed on its side, the shoot and root will emerge horizontally, but will quickly change their direction of growth. Within hours of germination, the shoot will bend to grow upward and the root will bend to grow downward.

The mechanism for this change in the direction of growth is similar to that for phototropism. Under the influence of gravity, auxin migrates to the lower side of the stem. Here, the hormone stimulates growth, causing the lower side to grow more quickly than the upper, bending the stem upward.

Remarkably, the opposite happens in roots. As in the stem, the auxin collects in the lower side of a horizontal root. However, root cells behave differently than stem cells do. Root cells elongate in response to lower levels of auxin in the upper surface, causing the roots to bend downward.