The changes in the environment to which the organisms respond and react are called stimuli.
The response to stimuli is a characteristic property of the living organisms. The working together of the various organs of an organism in a systematic manner so as to produce a proper response to the stimulus, is called coordination. Since plants are anchored by their roots, they cannot move in response to environmental stimuli. Instead, they change their growth pattern. For this reason, plants of the same species vary in body form. A growth pattern in response to an environmental stimulus is called a tropism. The plants use only the hormones for producing reaction to external stimuli. Plant hormones are also called phytohormones ('phyto' means 'plant'). Often, the response of a plant is governed by the interaction of two or more hormones.
Animals use both nervous system and hormones for coordination of their activities. Plants have no nervous system, so plants use only hormones for coordination. The plants respond to various stimuli very slowly by growing due to lack of nervous system. So, in most of the cases, the response of a plant to a stimulus cannot be observed immediately. It usually takes a considerable time to observe the effect of a stimulus on a plant.