Experiments by Charles and Francis Darwin demonstrated that the tip of a plant's shoot senses light, and that some kind of chemical signal travels from the tip to the growing region. The shoot responds to this signal by bending the toward the light.
In the 1920s, Dutch botanist Frits Went attempted to isolate the chemical signal from the tips of oat coleoptiles. This animation follows a recreation of his experiment and provides an opportunity for you to predict some of his experimental results.
Other experiments demonstrated that even when the tip is removed and a layer of agar placed between the tip and the base, the plant still grows toward the light. This suggests that the light-sensitive signal can travel through a permeable barrier, directing the plant's growth.
If the severed tip is replaced on the growing shoot so that it covers only half of the exposed cut, the side touching the tip grows faster than the other side, causing the coleoptile to curve, even in the dark. This demonstrates that the signal does not migrate across the shoot.
The Dutch botanist Frits W. Went wanted to find out if he could isolate this signal from the coleoptile tips. First, he removed the tips of oat plant coleoptiles. Next, he placed the detached tip onto a block of agar. After one hour, he discarded the tip and positioned the agar on decapitated coleoptiles in such a way that the block only covered half the exposed cut.
When Went placed the agar block on one side of the decapitated shoot, the shoot curved away from the agar as it grew. This demonstrated that some kind of hormonal signal had diffused into the agar from the coleoptile tips.
This hormone had then diffused into the growing coleoptile. The hormone, subsequently called auxin, caused the growing plant to curve, even in the absence of light. Went's experiment represented the first time that anyone had isolated a hormone from plants.
Experiments on growing seedlings showed that the plants grow toward light in response to a signal generated in the tip of the coleoptile. This signal, a hormone now known as auxin, moves down the dark side of the plant to stimulate faster growth on that side, causing the plant to curve toward the light.
Frits Went first isolated the hormone by placing severed coleoptile tips on blocks of agar. Auxin molecules diffused into the agar, and Went was able to stimulate growth in decapitated shoots by placing the auxin-containing agar onto the cut ends of the shoots. Went's experiment represented the first time anyone had isolated a hormone from a plant.