Plants exhibit phototropism when their shoots grow toward a light source. The first part of this video shows this phenomenon in a Mung bean seedling, Vigna radiata. You can see that the stem of this seedling grows toward the lamp located on the top left side of the screen (growth has been sped up 1,800 times). Experiments have shown that plants only respond to wavelengths of light in the blue end of the spectrum. This is illustrated in the second half of the video, where another Mung bean seedling is placed between two light sources: red light from the right and blue light from the left. Notice that the stem curves to the left toward the blue light as the plant grows. Credit: Courtesy of Graham R. Kent and Rebecca L. Turner, Smith College.
