Phototropism is (a kind of tropism) is a response of plant growth direction to one-sided lighting. There are positive (directed toward the source of light) phototropism, diatropism (like positioning of plate of leaf on the angle to the light) and negative phototropism - flexing of the plant body away from the light. The same plant may demonstrate positive or negative phototropism or none in dependence on intensity of light. Phototropism promotes even distribution of the leaves at the plant (to prevent their shading by each other), germination of seed out of soil etc. To sense the light there are specific photopigments on the surface of the plants. The animation demonstrates how in Darwins' experiment an auxin (the photopigment concentrated mainly on the tip of shoot) mediates a transmission of illumination influence and causes the flexing of plant. Auxin migrates from the light side to the shady side and thus on the shady side concentration of auxin increases. As a result cell stretching is stimulated and the plant flexes. Set the first group of switches in position "Darwins' Experiment" and select sequentially from the second group of switches "Lightproof cap", "Transparent cap" and "Collar" to make sure of this. Press the "Start" button to initiate animation. The animation is operated by the "Stop" and the "Reset" buttons.