Phytohormones (plant hormones) are plant-specific (endogenous) organic compounds that, as primary messenger substances (so-called signal molecules), control and coordinate the growth and development of plants. They are also referred to as growth regulators since they do not meet all the criteria of actual hormones. In addition to the real phytohormones, there are numerous other secondary plant substances that also have growth regulating effects, such as some phenolic compounds and steroids. However, these substances, by definition, do not belong to plant hormones.