USA Biology Olympiad (USABO) Practice Exam

What was the outcome when Darwin covered the tip of an oat coleoptile and illuminated it from one side?

1. The coleoptile grew toward the light

2. The coleoptile grew straight up

3. The coleoptile failed to grow toward the light

4. The coleoptile grew downward

The correct answer is: The coleoptile failed to grow toward the light

When Darwin covered the tip of an oat coleoptile and illuminated it from one side, the outcome illustrated the role of the coleoptile tip in sensing light, which is critical for directionality in plant growth. By covering the tip, the plant was unable to detect the light stimulus. Consequently, the coleoptile did not grow toward the light source because the phototropic response was effectively disabled. In the absence of light detection at the tip, the coleoptile grew uniformly without bending or orienting itself towards the light. This experiment helped establish that the tip of the coleoptile is essential for light perception, furthering the understanding of how phototropism works in plants. This confirms that the coleoptile’s growth direction is influenced by light exposure, and when this ability is obstructed, the expected growth response towards the light does not occur.