(hormone, phytohormone) Any substance that has a marked and specific effect on plant growth and that produces this effect when present in very low concentrations. Growth substances may stimulate plant growth by promoting cell division (see cytokinin
) or cell elongation (see gibberellin
) while others, such as *abscisic acid
, inhibit certain developmental processes. The term includes substances produced within the plant and also artificial, often structurally related, chemicals that have similar effects. Endogenous growth substances are often produced in a particular region, for example *auxin
is synthesized in shoot apices. They are then transported from these regions and take their effect at sites far removed from the point of production. Normal growth is only achieved when there is a correct balance of the various growth substances. Abnormal growth and even death may result from unusually high or low concentrations of a growth substance. This reaction is exploited in the use of certain weedkillers that contain synthetic auxins.