The hormone which controls plant growth.
Phototropism (response to light)
Auxin is produced at the tip of the shoot. It is stimulated on the shady side of the root. The cells on this side elongate and the shoot bends towards the light.
Geotropism (response to gravity)
Auxin stimulates cells to elongate on lower side of shoot, which therefore bends up, away from gravity.
Auxin slows down cell growth on lower side of root, meaning cells on upper side grow quicker and root bends down, towards gravity.
Hydrotropism (response to moisture)
Auxin is produced on one side of the root so cells elongate to bend towards moisture. The side auxin is stimulated on can change according to where the moisture is.
Ripening with auxin
Auxin is sprayed on unpollenated flowers, which then ripen without any seeds or pips.
Auxin root powder
Plant cuttings are dipped in powder containing auxin, so the cutting is stimulated to grow roots.
Weedkillers containing auxin are sprayed on broadleaved weeds - they do not work on any others. The auxin makes them grow too fast and die.