plant hormone or substance that promotes or regulates the growth and development of plants. Produced at sites where cells are dividing. primarily in the shoot tips. Auxin-like compounds may be synthetically produced.

Merriam-Webster Online Dictionary
auxin (noun)
any of various usually acidic organic substances that promote cell elongation in plant shoots and usually regulate other growth processes (as root initiation) as
a) - indoleacetic acid
b) any of various synthetic substances (as 2,4-D) resembling indoleacetic acid in activity and used especially in research and agriculture
- plant hormone
auxin (Wikipedia)
Native auxins
Skeletal structure diagram
Indole-3-acetic acid (IAA) is the most abundant and the basic auxin natively occurring and functioning in plants. It generates the majority of auxin effects in intact plants, and is the most potent native auxin.

There are four more endogenously synthesized auxins in plants.
All auxins are compounds with aromatic ring and a carboxylic acid group:
For representatives of synthetic auxins, see: Auxin § Synthetic auxins.

Auxins (plural of auxin /ˈɔːksɪn/) are a class of plant hormones (or plant-growth regulators) with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essential for plant body development. The Dutch biologist Frits Warmolt Went first described auxins and their role in plant growth in the 1920s.

Kenneth V. Thimann (1904-1997) became the first to isolate one of these phytohormones and to determine its chemical structure as indole-3-acetic acid (IAA). Went and Thimann co-authored a book on plant hormones, Phytohormones, in 1937.

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