A plant without access to light will grow weak and die. Whereas most organisms on Earth obtain their energy by consuming other organisms, plants are unique in their need for the sun. Through plants, the energy from the sun becomes available as chemical energy to all life on Earth.
If you've ever sprouted seedlings without adequate light, you know that they become spindly and stretched until they eventually fall over and die. As this shows, plants need light to live. They produce their own energy using sunlight and a unique process called photosynthesis. For this reason, plants are called primary producers.
Photosynthesis is the plant's most important reaction to light because it supports the plant's life and governs much of its behavior. When light strikes special cell structures called chloroplasts, it initiates a chemical reaction that produces energy that the plant uses to grow and form flowers, seeds and fruit with. When foraging animals consume plants, they make use of these sugars, carrying the sun's energy up the food chain.
Plants are stuck in one place, so they cannot get up and move to a new spot when light becomes scarce. Because light is essential for plants to survive, they have developed many behavioral adaptations that allow them to maximize their exposure to light and, therefore, produce more energy.
Phototropism is a dramatic plant response to light, observable even in ordinary houseplants. When one side of a plant receives more light than the other--such as when plants are placed in a sunny window--a hormone called auxin stimulates growth on the darker side of the plant, causing the plant to lean towards the light. In some species, this response is so dramatic that the plant can reorient itself in as little as a day. For example, Arctic poppies will rotate 360 degrees every day as their flowers follow the Arctic sun in its course around the sky.
Because seeds are generally covered with soil when they germinate, germination is not usually a light-dependent process. However, some seeds have areas that are sensitive to light and likely help the plant to determine when it is oriented properly in the soil for the greatest chance of successful growth. Other seeds only germinate when light is present, especially species that tend to grow in the deciduous forest. These species germinate during the early spring, triggered by light filtering through the underdeveloped canopy. By the time the canopy thickens for summer and light no longer reaches the forest floor, the young trees have established themselves and the woodland wildflowers have completed their life cycle, setting seeds for the next generation.