Phototropism is a plant's growth response to light.
Plants need light to grow. Usually, plant shoots and stems exhibit a positive reaction to light (they grow towards the light source).
The ability to grow towards sunlight gives the plants an evolutionary advantage. More sunlight energy means more photosynthesis can be done by chloroplasts. Therefore, more carbon dioxide is converted into organic compounds. And plants need these organic compounds in order to grow and to reproduce.
Discover other important plant tropisms:
Plant roots, on the other hand, show a negative reaction to light and grow in the opposite direction into the earth where it is dark. Yet, gravitropism (positive gravitropism) is the major factor for the growth direction of roots.
Also, some plants grow away from light, i.e. climbing plants when searching and turning for a climbing support (thigmotropsim). This is especially true for certain tropical vines: They grow away from light until they reach a tree trunk, which they use for growth support. This specific growth direction is actually called skototropism.
After reaching a stem the vines grow fast and easily up towards sunlight.
Why do we humans see light? We have certain photoreceptors in our eyes that can perceive light, the so-called rod and cone cells.
Plants do the same thing! They also have photoreceptors. These photoreceptors are able to detect light and gather information through the light properties. The response to the stimulus is a change in the directional plant growth.
What Causes this Plant Growth Process Stimulated by Light?
I am sure that you have observed houseplants growing towards the window and present their leaves to the sunlight. Studies have shown that plant hormones of the class of auxins play a key role in this directed growth process. Auxin is released into the cells on the plants shady side. There, the hormone stimulates the cell walls to elongate. Also, growth is inhibited on the plant's sunny side. This lets the plant bend and grow towards the light source.
Of course, in the wild, plants do the same thing. All plants try to reach as much light as they can tolerate and stay in their light sensitivity optimum range. Shade loving plants and sun loving plants have obviously different light preferences!
A specific positive phototropism is the diurnal reaction of light intensity and direction: this is called heliotropism.