What is the name of the growth response of plants towards a light source?

The growth response of plants towards a light source is called phototropism. Most shoots show positive phototropism, bending toward light so their leaves can capture more energy for photosynthesis.

The bending results from uneven growth on opposite sides of a stem. In many shoots, the hormone auxin becomes concentrated on the shaded side, where it encourages cells to elongate. That side grows faster, curving the shoot toward the light. Blue-light receptors called phototropins help the plant detect the direction of illumination.

Phototropism is one kind of tropism, a directional growth response to an external stimulus. It is often confused with gravitropism, which responds to gravity, and hydrotropism, which responds to water. Roots and shoots can show different responses: shoots commonly grow toward light, while some roots respond negatively to light.

The phenomenon was studied extensively in the nineteenth century, including experiments by Charles Darwin and his son Francis on grass seedlings. Older sources sometimes use heliotropism, but phototropism is the standard modern term.