Plant hormones do not work on animals because Plant hormones were developed by plants as one way of assuring their survival. Since a plant cannot move away from a threatening situation, it is important that it have an internal messenger system that ensures that the entire plant is able to react in a proper way to its environment. This role is filled by the messenger plant hormones. Unlike animals, plants have no specialized organs designed solely for hormone synthesis and secretion . Leaves, stem tips, root tips, flowers, seeds, and fruits all produce hormones. Most plant hormones are functional at very low concentrations.