Auxins stimulate cell elongation in shoot tips, embryos, young leaves, flowers, fruits, and pollen.
Gibberellins stimulate cell division and elongation but act more slowly than auxins. Cytokinins stimulate mitosis in actively developing plant parts. Ethylene speeds ripening. Abscisic acid inhibits the growth-inducing effects of other hormones. A tropism is a growth response toward or away from an environmental stimulus, usually caused when different parts of an organ or structure grow at different rates.