AP BIOLOGY

Detailed explanation-1: -Roots grow downward, or towards the center of Earth, and away from light. These responses to external stimuli are called tropisms. Plants’ growth response to gravity is known as gravitropism; the growth response to light is phototropism.

Detailed explanation-2: -The reason plants know which way to grow in response to gravity is due to amyloplasts in the plants. Amyloplasts (also known as statoliths ) are specialized plastids that contain starch granules and settle downward in response to gravity. Amyloplasts are found in shoots and in specialized cells of the root cap.

Detailed explanation-3: -When a stem is placed horizontally, the bottom side contains more auxin and grows more-causing the stem to grow upwards against the force of gravity. In a root placed horizontally, the bottom side contains more auxin and grows less-causing the root to grow in the direction of the force of gravity.

Detailed explanation-4: -Graviperception by plant roots is believed to occur via the sedimentation of amyloplasts in columella cells of the root cap. This physical stimulus results in an accumulation of calcium on the lower side of the cap, which in turn induces gravicurvature.