When you expose lettuce seeds to different concentrations of salt solution, you can watch a clear pattern unfold. Seeds placed in high-concentration NaCl solutions germinate less often, and the roots that do grow come in shorter. As the dose changes, the biological response changes with it — predictably, measurably. This is what makes the lettuce bioassay useful: a living organism that either sprouts or it doesn't gives you a clean signal about how a substance affects life at different amounts.