Controlled Environments offer a unique opportunity to study both the synthesis of ethylene and the sensitivity of plants to ethylene. Our recent studies indicate that plants are 10,000 times more sensitive to ethylene than humans are to carbon monoxide. Low levels of atmospheric ethylene (50 ppb) have a minimal effect on photosynthesis, but significantly reduce the rate of leaf expansion. Extremely low levels of ethylene (below 20 ppb) inhibit pollination and seed set. Ethylene is continuously synthesized throughout the life cycle. Small changes in synthesis rates can cause large changes in plant development.