Imagine you are reading in a room as night falls. When the light becomes too dim to see, you can reach up to switch on a lamp or move to an area with more light. Because plants are literally rooted in place, they cannot relocate when conditions are less than optimal. Tropisms are a broad category of plant behaviors that involve plants detecting optimal conditions and adjusting their growth, so that they move in that direction. Phototropism is the movement of plants toward light.
Although many organisms prefer light, plants require it in order to survive. Plant metabolism is based in a process called photosynthesis, where plants use energy from the sun to synthesize chemical energy in the form of sugar. Phototropism allows plants to maximize their exposure to light--and, therefore, their production of energy--by growing toward a light source.
In 1834, botanist Augustin de Candolle noted that plants growing in a dark basement with a single row of windows always angled their growth toward the windows, disproving the theory that plants grew toward air sources, not light. In 1880, Charles Darwin and his son Francis discovered that the new growth at the tips of plants is the part that responds to light. When they covered the tips, the plants did not exhibit phototropism. Later studies in the early 20th century revealed that a chemical stimulated phototropism in the plant.
Phototropism requires plants to perceive a light source and respond to it. Plants perceive a light source when light activates the enzyme phototropin. Phototropin is sensitive to blue wavelengths of light, and its activity increases when stimulated by them, triggering the phototropic response.
During summers in the Arctic, the sun never fully sets, but moves in a circle around the sky. In a dramatic example of phototropism, the Arctic poppy flower rotates a full 360 degrees each day, following the sun. As E.C. Pielou explains in her book "A Naturalist's Guide to the Arctic," this movement isn't caused by a bending or twisting of the stem but by uneven growth on the part of the stem that receives the least light. These cells grow faster, bending or turning the plant toward the light. This explains the Darwin's observations as well that phototropism occurs in young plant tissue.
The chemical responsible for the uneven growth that causes phototropism is called auxin, a plant hormone that stimulates growth. Auxin accumulates on the darkened side of the stem, causing the cells there to grow faster, pushing the plant away from the shade and into the light.