Auxins are plant growth hormones. They control growth near the tips of shoots and roots. Auxin is prouduced in the tips and diffuse backwards to stimulate the cell elongation process which occurs in the cells just behind the tips. If the tip of a shoot is removed, no auxin will be available and the shoot may stop growing. On the other hand, the tips also produce subtances which inhibit the growth of side shoots. Therefore hedge trimming promotes bushier hedges.

Auxins change the direction of root and shoot growth. Note that in shoots they promote growth, while in roots they inhibit it.

1. Shoots bend towards the light. When a shoot tip is exposed to light, more auxin is supplied on the side in the shade, causing that side to grow faster and bend towards the light.
2. Shoots bend away from gravity. Gravity causes an uneven distribution of auxin in the shoot, with more on the lower side. This causes the lower side to grow faster, bending the shoot away from gravity.
3. Roots bend towards gravity.In roots, the uneven distribution of auxin due to gravity causes the top side to grow faster due to auxin inhibiting growth in roots.
4. Roots bend towards moisture. An uneven degree of moisture either side of a root will cause more auxin to appear on the side with more moisture. This inhibits growth on that side and causes the root to bend towards moisture.

At high concentrations, auxins are sometimes used as herbicides. The auxins completely disrupt the growth patterns of the plants they are designed to effect and this results in death for the plant.

An auxin is any of several hormones that play a role in many aspects of plant growth and development. 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 the integral dioxin contamination, 2,4,5-T has been implicated in leukemia, miscarriages, birth defects, liver damage, and other diseases.

Some of this writeup is based on work I did for the science dictionary at

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