Define phototropism and give one example.

Phototropism is a biological phenomenon observed in plants, where they exhibit growth or movement in response to light. Specifically, phototropism refers to the directional growth or curvature of a plant's stem, leaves, or other structures in response to the direction of a light source. This growth response is controlled by the hormone auxin, which accumulates on the side of the plant that is away from the light source, causing cells to elongate and the plant to bend toward the light.

Example: Sunflowers (Helianthus annuus) are a classic example of phototropism. In the morning, their young flower buds and leaves are typically facing east, as they grow toward the direction of the rising sun. Throughout the day, they follow the sun's movement across the sky, a behavior known as heliotropism. This maximizes their exposure to sunlight, which is essential for photosynthesis and overall plant growth.