Plant cells are in constant chemical communication with one another and with their environment. They recognize and respond to stimuli of many kinds, using a system of chemical messengers that receive and transmit the stimuli via ordinary body cells (unlike the highly specialized cells of animal nervous systems). Control of the plant system apparently resides in the genes of each cell, which are turned on and off by the chemical messages they receive. The response may be stimulatory (initiating cellular division and enlargement, for example) or inhibitory (such as stopping a metabolic process).
The chemical messengers are hormones, organic substances manufactured in small amounts in one tissue and usually transported to another where they initiate a response. (A few act in the tissues where they are produced.) The hormone molecule itself carries little information and produces a reaction only when it binds to appropriate receptor molecules at the response site.
Plants, in comparison to animals, have both fewer hormones and fewer kinds of responses. Plant hormones, however, usually act in combination, thus producing more varied responses than if acting individually. The same hormone also can produce different effects when acting in different tissues or in different concentrations in the same tissue. The developmental stage of the plant additionally determines what effects the hormone activates. Growth and development depend upon a successful coordination of the activities of hormones, not just the presence or absence of individual ones.