Tropism - plant movement response in which the direction of the response is dependent on the direction of the stimulus (e.g., phototropism and gravitropism, thigmotropism)
Nastic response - movement response that is independent of the direction of the stimulus (e.g., sleep movement of leaves)
Studies of tropisms led to discovery of auxin.
Darwin used coleoptiles to study phototropism. His experiments led him to conclude that "when seedlings are freely exposed to unilateral light, some influence is transmitted from the upper to the lower part, causing the latter to bend". His experiments and those from Boysen Jensen (the gelatin block treatment) are outlined in the following images. These experiments demonstrated that the site of light perception for phototropism is the coleoptile tip and that a "diffusible substance", called auxin, that moved from the tip caused differential growth of cells below the tip. This pioneering work set the stage for the discovery of the plant hormone, indole acetic acid.
In the 1920's Fritz Went, Kenneth Thimann and others attempted to isolate the "diffusible substance" that was responsible for causing phototropism. They developed a sensitive bioassy system based on the cruvature response and were able to measure the amount of the substance based on it biological activity and they were able to follow the activity thorugh various chemical extraction procedures. The bioassy is shown below.
Using the bioassy and the experiments shown below, it was shown that unilateral light caused the "diffusible substance" to be laterally transported in the tip before the substance moved down to the elongation zone in a polar fashion.
Eventually, researchers were able to use the bioassay to purify enough of the "diffusible substance" for it to be chemically identified.
Click here for Phototropism Part II (not ready yet)