USA Biology Olympiad (USABO) 2025 – 400 Free Practice Questions to Pass the Exam

When a seedling is placed in a window that faces east, it will receive direct sunlight primarily from that direction in the morning. Plants have a growth hormone called auxin that promotes cell elongation. Auxin distribution in plants is affected by light, and it typically accumulates on the side of the plant that is away from the light source.

In this scenario, since the light is coming from the east, the auxin will accumulate on the west side of the seedling. This uneven distribution causes the cells on the west side to elongate more than those on the east side, resulting in the plant bending or growing towards the light source. This phenomenon is known as phototropism, where plants grow in response to light.

Thus, when a seedling is placed in an east-facing window, the auxin will accumulate on the west side, allowing the plant to grow toward the light.