Gravitropism
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Tropism is a growth response between a plant and an external stimulus. The stimulus could be weather, touch, time, gravity or light. A positive response is indicated by growth toward a stimulus and a negative response is indicated by growth away from the stimulus.

Gravitropism is a plant's growth response to gravity. Typically roots grow down into the ground, and stems and leaves grow up above the ground. If one were to place a plant on its side, it would begin to bend in an upright position. This occurs because of the plant hormone also called auxin that plays a part in gravitropism. When the plant is laid on its side, the auxin concentration increases along the lower sides of the roots and stems of the plant. The auxin stimulates cell elongation on the stem which makes the stem bend up toward the sky. The auxin prohibits cell elongation in the roots which causes the roots to bend down into the soil. Gravitropism allows the plant to respond to gravity no matter what position the plant is in.

Sources:
Biology of Plants, 6th ed. Raven, Peter H., Everert, Ray F., Eichhorn, Susan E. Worth Publishing 1999.

Botany: An Introduction to Plant Biology. Mauseth, James D. Jones and Bartlett Publishers. Sudbury, Massachusetts 1998.