3 Answers
The movement of plant in response to light is called phototropism. Stem shows positive phototropism as follows: When growing plants detect light, a hormone called auxin, synthesised at the shoot tip, helps the cells to grow longer. When light is coming from one side of the plant, auxin diffuses towards the shady side of the shoot. This concentration of auxin stimulates the cells to grow longer on the side of the shoot which is away from light. Thus, the plant appears to bend towards light.
Phototropism is a directional growth which occurs in response to unidirectional exposure to light. Phototropic movement is generally caused by increased auxin on the dark side and lesser auxin on the illuminated side. Due to the presence of more auxin, the part of the plant stem in the dark grows faster, causing it to bend towards the source of light.
Environmental triggers, such as light or gravity will change the directions that plant parts grow in. These directional or tropic, movements can be either towards the stimulus or away from it. So in two different kinds of phototropic movement, shoots respond by bending towards light while roots respond by bending away from it.