Hint: Just like animals, plants also have mechanisms of chemical signalling within the body. These have short and long-term effects and are known as phytohormones. They are synthesised in quite low concentrations.
There are several types of plant hormones responsible for coordinating growth and development of the plants as well their reactions to environmental stresses.
Auxins like IAA are a type of plant growth hormone that is produced in the tips of the roots and shoots. They are responsible for promoting cell division, and thereby growth of the shoot and roots. Auxins also regulate phototaxis in plants. When a plant continuously receives sunlight from one direction, the side opposite to that undergoes more cell division allowing the plant to bend towards the light.
Gibberellins also regulate growth processes like germination and stem elongation, dormancy and flowering, as well as senescence. There are any different gibberellins (Gas), and GA3 has great commercial importance. Also known as gibberellic acid, it is used in brewing and horticulture. Very rarely do GAs promote growth by cell-division.
Ethylene is a gas that functions as a plant hormone. It plays a vital role in fruit ripening and senescence. It is also involved in determining the sex of a flower in many plants. It has also been seen to help regulate the production of other hormones.
Cytokinins work along with the auxins. They stimulate cell division and also the differentiation of axillary buds. Increased amounts of cytokinin will promote branching, while more of Auxin promotes stem elongation.
Abscisic acid or ABA is also involved in seed dormancy and senescence. Plants faced with stresses like drought and flooding respond with increased leaf senescence. ABAs actions are antagonistic to the gibberellins.
Note: Different plant hormones have different functions, but these work in coordination with each. Three of them are involved in plant growth and development, but their method of action would be slightly different.