Can a lettuce seed tell you how toxic a solution is? In a reference toxicity test, you expose a living organism to different doses of a chemical. Then you measure the response. Lettuce seeds work well because they either sprout or they do not. You prepare five salt (NaCl) solutions at different concentrations. You place seeds on moist filter paper in petri dishes soaked with each solution. A set of control dishes gets plain deionized water. After five days in the dark, you count how many seeds sprouted. You also measure each root to the nearest millimeter. At high salt concentrations, fewer seeds germinate and roots grow shorter. This dose-response pattern shows whether lettuce seeds respond in a predictable way. A bioassay is a test that uses living things to detect harmful substances.