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(Science: plant biology) a group of plant growth substances (often called phytohormones or plant hormones), the most common example being indole Acetic Acid (IAA), responsible for raising the ph around cells, making the cell wall less rigid and allowing elongation.

auxins include indoleacetic acid, phenylacetic acid, and 4-chloro-indoleacetic acid. Commercially, auxins are used to promote root growth, to promote uniform flowering, and to set fruit and prevent premature fruit drop. Synthetic auxins such as 2, 4-D and 2, 4, 5-T have been used as herbicides, broad-leaved weeds like dandelions are much more susceptible to auxins than narrow-leaved plants like grass and cereal crops.

The defoliant agent orange was a mix of 2, 4-D and 2, 4, 5-T. 2, 4-D is still in use and is thought to be safe, but 2, 4, 5-T was more or less banned by the EPA in 1979. The dioxin TCDD is an unavoidable contaminant produced in the manufacture of 2, 4, 5-T, as a result of the integral dioxin contamination, 2, 4, 5-T has been implicated in leukaemia, miscarriages, birth defects, liver damage, and other diseases. A plant hormone that promotes root formation and bud growth.An auxin is a growth regulating substance that can be found in the meristems of plants. The concentration of auxin has the deciding effect on how much (and where) growth occurs in a plant.