Seeds are then tested for their ability to germinate when moisture is limited. To achieve this in the laboratory, we simulate decreased water potential using the polymer polyethylene glycol (PEG). PEG influences the osmotic potential of solutions by binding to water molecules, which are then unable to be absorbed by plant cells. This information is useful when re-seeding ecosystems as it tells us whether we might need to provide extra water to seeds of some species to increase germination success. Following this test, a species is designated as having a ‘very high’, ‘high’ ‘moderate’ or ‘low’ sensitivity to moisture stress.