Scientists were interested in how plants respond to light; if plants don’t have eyes, how do they sense where light is and which way they should grow? It is a common observation that plants grow toward light, but for a long time no one knew why.
One of the first people to experiment with this concept was Charles Darwin, who along with his son, Francis, was interested in figuring out how plants respond to stimuli (in this case, light). They noticed that coleoptiles, which are sheaths that protect grass stems as they germinate, bend toward light. They tried covering the coleoptile with foil and found that when covered, the coleoptile didn’t bend. When uncovered, it bent again! From this the Darwins concluded that the tip of the grass coleoptile senses light.
Even though it doesn’t seem very exciting now, in the 1800s this was just as scandalous as Lady Gaga’s meat dress. The idea that plants could do something as brilliant as respond to their environment was shocking in an age when Man was exerting control on all things wild.
Later work by another scientist, Frits Went, determined that the signal responsible for bending toward light was a mobile chemical, and Went went ahead and gave it the name auxin. These days, auxin is sometimes referred to by its chemical name, indoleacetic acid (IAA).
Auxin does a couple different things in a plant, but its main role is to work with another type of hormone (cytokinins) to stimulate elongation of stems. If auxin is helping cells elongate, it is likely found in a place where a lot of new cells are forming. Where would that be? The shoot apical meristem, of course! The shoot apical meristem is a major source of auxin, but not the only one. Developing seeds also produce auxin, which leads to fruit development. When fruits such as tomatoes are grown inside greenhouses where there are no insect pollinators, synthetic auxins are used to help fruits develop normally.
Another commercial use of auxin is in the vegetative propagation of plants from cuttings. Instead of planting seeds, people can grow some plants by just cutting a leaf or stem; spraying the detached leaf or stem with auxin induces root production, and a whole new plant is formed.