Charles Darwin performed experiments on plant movements with his seventh child, Francis Darwin, which they wrote up as joint authors in 1880 (Francis was a young man at the time). Every year, thousands of children and budding scientists repeat these experiments:
- Shine a light on a plant from just one source, and observe as plants orientate their growth to ‘face’ the light.
- Adorn growing shoots with a ‘cap’ that is impermeable to light and observe as they grow straight, even as light hits the open stem.
The tip of the plant senses the direction of the light, and this information is transmitted to the growing stem to direct growth behaviour. That transmission is performed by auxins, the first of which was isolated in the 1930s.