English: The process of chemotaxis can be demonstrated using a capillary tube assay(shown above). The motile prokaryotes can sense chemicals in their environment and change their motility accordingly. When no chemicals are present, movement is completely random. When a repellent or attractant chemical is present, the motility changes; runs become longer and tumbles become less frequent so that the net movement towards or away from the chemical can be achieved. The net movement can be seen in the beaker, where the bacteria accumulate around the attractant, and away from the repellent.