In common with all living organisms, plants acquire the materials they need for energy and growth from their surroundings. In the case of plants, this acquisition usually has to be made against a concentration gradient. For land plants, tissue water must be retained against gradients of atmospheric and soil water potential, which can vary from small in humid tropical forests to very large in hot, dry deserts. The concentrations of many essential ions in plants are often a thousand-fold greater than those in the soil solution from which their roots extract them. Since plants invest considerable amounts of metabolic energy to acquire and retain external resources in the face of usually limited supply, it could be said that stress is a normal condition of plants even in relatively benign environments.