Seedlings grown in a dark closet will outgrow their counterparts grown in the light, stretching to find light, then suddenly dying. Houseplants will grow toward a lighted window until the entire plant leans to one side. Light is not a comfort or convenience for plants but a necessity, driving the creation of energy that, in turn, allows them to grow.
Plants are defined by their ability to use sunlight to produce food. Other organisms acquire energy through consumption--whether grazing, hunting or the drive-thru window--but plants turn their leaves to the sun, absorb light and, from that, create the energy needed to power their life processes, including growth. (See References 1)
The sun showers the Earth constantly with energy in the form of light, but that energy cannot be used in the form in which it arrives. Living organisms need energy packaged as sugar. The metabolic process of photosynthesis solves that quandary. Six molecules of water and six molecules of carbon dioxide contain all that is needed to build a glucose molecule but require energy to break and reassemble the chemical bonds. During photosynthesis, light excites molecules in special plant cell structures called chloroplasts. That energy kicks off the process of photosynthesis, generating glucose from water and carbon dioxide. (See References 2)
One of the most important uses of the energy produced during photosynthesis is growth. Plants grow through cell division. The contents of a cell duplicates itself, and the cell splits into two. However, this process requires a lot of energy. Cells use energy in two phases during growth: to replicate their DNA and to actually divide the cells. Photosynthesis provides plants with the energy they need to undertake these processes. (See References 3)
Observing how plants behave in different light environments reveals the effect light has on growth. On his website, retired biology professor John W. Kimball shows two seedlings side by side, one that has been grown in the dark and one that has been grown normally. The plant grown in the dark is significantly larger, a phenomenon known as etiolation. During etiolation, a plant uses nutritive contents inside of the seed to grow rapidly, literally stretching in search of light. When the seed contents are depleted, the plant dies. (See References 4) During a similar process, called phototropism, plants will perceive and grow toward a light source in order to maximize the contact between their leaves and the light. (See References 6)
On his website, Dr. Kimball offers further insight on how light affects plant growth. Studies of etiolation and phototropism show that plants experience spikes in growth not in response to light but in response to darkness. While normal growth requires energy and, therefore, light, above-average growth is spurred by the presence of darkness. This process causes the elongation seen in etiolation and the growth toward light seen in phototropism, helping plants to reach more light so that they can resume normal growth. (See References 5)