Growth hormone (auxin) is a class of endogenous hormones containing an unsaturated aromatic ring and an acetic acid side chain, abbreviated as IAA, whose chemical essence is indoleacetic acid. In addition, 4-chloro-IAA, 5-hydroxy-IAA, naphthalene acetic acid (NAA) and indole butyric acid are growth hormone analogues. Growth hormone was the first plant hormone to be discovered. The most important chemical in growth hormone is 3-indoleacetic acid. It is used in agriculture to promote the rooting of cuttings, which is very effective.