Given a scientific passage and an answer, generate a question for the given answer.

Passage: Vertebrates have a closed circulatory system with a heart. Blood is completely contained within blood vessels that carry the blood throughout the body. The heart is divided into chambers that work together to pump blood. There are between two and four chambers in the vertebrate heart. With more chambers, there is more oxygen in the blood and more vigorous pumping action. Answer: chlorophyll
Leaves respond to these environmental stimuli by producing less what?