Plants require light to survive because they produce their own food from sunlight in the process called photosynthesis. However, because plants cannot simply uproot and move to a location with ideal light, they will adapt to limited light. Observing plant behavior when receiving light from one side reveals the process of phototropism.
Most organisms on Earth acquire energy through consumption: They eat plants or other animals, and their bodies digest and absorb energy-containing compounds. Plants play a different role, however. Most plants are unable to consume other organisms, so they must produce their own energy. Through photosynthesis, plants combine carbon dioxide with water and, using energy from the sun, power a chemical reaction that rearranges these molecules to produce glucose, a sugar that they can use for food. For this reason, access to light is essential for plant survival.
Farmers and gardeners had always observed that plants will lean toward a light source but, prior to the 18th century, the cause was assumed to be a difference in air currents, not light. Augustin de Candolle first proposed in 1809 that plants grew unequally when given light on only one side. This plant behavior is called phototropism, meaning growth toward light.
Tender new plant growth is more elastic and adapts more readily to changing conditions. When a plant receives light on only one side, receptors inside the plant, called phototropins, detect the presence of light. These receptors shut down production of a hormone called auxin, which then flows only on the dark side of the plant. Auxin stimulates growth, causing the dark side of the plant to grow slightly faster than the light side. The faster growth of the darkened side pushes the plant over, to lean into the light.
Photosynthesis powers plant life. Without it, plants don't have energy for growth and metabolic function, so it is in the plant's best interest to maximize its exposure to light. Phototropism is one way for plants to do that, using chemical reactions driven by light differentials and grow in such a way to get as much exposure as possible to the light source.
You can observe phototropism just by observing how houseplants press themselves again a sunny window. In her book "A Naturalist's Guide to the Arctic" E.C. Pielou offers a dramatic example of phototropism at work. Many Arctic flowers attract pollinating insects by maintaining their flowers at a slightly warmer temperature than the outside air. To achieve this warmth, the Arctic poppy rotates to constantly face the sun. During Arctic summers, the sun does not set but circles the sky, keeping close to the horizon, meaning that the Arctic poppy rotates 360 degrees each day, using the mechanism of phototropism.