AP BIOLOGY

Detailed explanation-1: -Plants respond directly to Earth’s gravitational attraction, and also to light. Stems grow upward, or away from the center of Earth, and towards light. Roots grow downward, or towards the center of Earth, and away from light.

Detailed explanation-2: -Types of tropisms include gravitropism (gravity), phototropism (light), and thigmotropism (touch). Plant roots always grow downward because specialized cells in root caps detect and respond to gravity. This is an example of a tropism. A tropism is a turning toward or away from a stimulus in the environment.

Detailed explanation-3: -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-4: -Plants respond to their environment. They grow toward light. Plant leaves bud and seeds germinate when the temperature is right. Their roots and stems grow in certain directions in response to the pull of gravity.