MACHÁČKOVÁ, Ivana, ZAŽÍMALOVÁ, Eva, GEORGE, E.F. Plant Growth Regulators I: Introduction; Auxins, their Analogues and Inhibitors. In: GEORGE, E.F., HALL, M.A., DE KLERK, G.J., eds. Plant Propagation by Tissue Culture 3rd Edition. Dordrecht: Springer, 2007, S. 1-501. ISBN 978-1-4020-5004-6

Some chemicals occurring naturally within plant tissues, have a regulatory, rather than a nutritional role in growth and development. These compounds, which are generally active at very low concentrations, are known as plant hormones (or plant growth substances). Synthetic chemicals with similar physiological activities to plant growth substances, or compounds having an ability to modify plant growth by some other means, for example polyamines, are usually termed plant growth regulators. Some of the natural growth substances are prepared synthetically or through fermentation processes and can be purchased from chemical suppliers. There are several recognised classes of plant growth substances. Until relatively recently only five groups were recognised namely: auxins, cytokinins, gibberellins, ethylene, abscisic acid.