Plant hormones, also known as phytohormones, are signal molecules produced within plants that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, pathogen defense, stress tolerance, and through to reproductive development. Unlike in animals, plant hormone action is not confined to specialized cells or glands. Instead, each plant cell is capable of producing hormones. Plant hormones also differ from animal hormones in that their production is not confined to particular locations. Plant hormones are not always transported to other parts of the plant, and may also act on the cell or tissue where they are produced.