You might have observed the tendrils of plants growing towards a support. Can you imagine how is it happening? Would you think it is responding to a stimulus?
Both plants and animals react to various stimuli around them. But the method of responding to stimuli is not similar in plants and animals. Higher animals respond to stimuli because they have a nervous system and an endocrine system. Plants do not have a well-defined nervous or endocrine system. They do have some mechanism of control by means of some chemicals or hormones.
Plants can sense the presence of stimuli like light, heat, water, touch, pressure, chemicals, gravity etc. The hormones present in the plants called phytohormones (phyto means plant) control responses towards the stimuli mentioned above. Phytohormones coordinate the activities of the plant usually by controlling one or the other aspect of the growth of the plant. So plant hormones are also called growth substances. Some major plant hormones and their action are given in the following table.
| Hormones | Uses |
|---|---|
| Auxins | cell elongation and differentiation of shoots and roots |
| Cytokinins | promote cell division, promotion of sprouting of lateral buds, delaying the ageing in leaves, opening of stomata. |
| Gibberellins | germination of seeds and sprouting of buds; elongation of stems; stimulation of flowering; development of seedless fruits, breaking the dormancy in seeds and buds. |
| Abscisic acid | closing of stomata; seed dormancy,promoting aging of leaves. |
| Ethylene | ripening of fruit |
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