In fact, each sphere represents a sector of existence on the planet that is at once clearly defined and virtually inseparable from the others. Each is an open system within the closed system of Earth, and overlap is inevitable. For example, the seeds of a plant (biosphere) are placed in the ground (geosphere), from which they receive nutrients for growth. In order to sustain life, they receive water (hydrosphere) and carbon dioxide (atmosphere). Nor are they merely receiving: they also give back oxygen to the atmosphere, and by providing nutrition to an animal, they contribute to the biosphere.