The idea behind a reference toxicity test is that the test organism, in this case lettuce seeds, will respond in a predictable manner to varying concentrations of a particular chemical compound. At some threshold concentration, all of the test organisms will be killed (or in this case, none of the lettuce seeds will sprout). In solutions that are more dilute, some level of inhibition will occur in seed germination and/or radicle length. If the concentration is low enough, no response will be detectable.