Unlike most familiar life on Earth, plants do not eat but, rather, synthesize their food using carbon dioxide, water and energy from the sun. For this reason, plants require adequate exposure to light, and they have evolved mechanisms that allow them to maximize their exposure in conditions where light is limited. When light comes from only one direction, you can observe one of the plant kingdom's more impressive evolutionary adaptations: phototropism.
Phototropism is the tendency of plants to modify their growth so that they extend or lean toward a light source. If you've ever grown houseplants in a bright, sunny window, you've probably observed phototropism as, over time, your plants lean and begin to press into the glass. Because each plant cell contains the machinery needed to convert sunlight to energy, exposing as much of the plant as possible to the sunlight allows the plant to maximize its energy production.
Farmers and horticulturists had always observed that plants grew lopsided when exposed to light from a single direction, but they always credited increased air flow -- not light -- as the cause of the behavior. Professor von Sengbusch of the University of Hamburg describes how early experiments exposing plants to light on one side and air on another allowed botanists to realize that plants were growing toward light and doing so through unequal growth between the two sides of the stem.
As biology professor John W. Kimball points out, when you observe plants, you are observing two different types of phototropism. Positive phototropism is the more impressive form, where plants grow dramatically toward a light source. In some species, such as the Arctic poppy, phototropism causes the plant to make a 360 degree rotation each day. Negative phototropism is less dramatic but no less important and occurs when plant roots grow away from a light source, tapping into the water and nutrients found in deeper levels of the soil.
Plants lack nervous systems and sensory organs, so how do they know which direction a light source is coming from? Embedded in each plant cell are special receptors call phototropins. These receptors detect blue wavelengths of light, present in sunlight. As Professor Kimball explains, growth stimulation occurs in the darkened part of the plant, where the phototropins don't detect a light source. A plant hormone called auxin moves into the darkened side of the plant, causing that side to grow faster and bending the plant toward the light.
Plant cells generally have a rigid structure compared to animal cells, so phototropic growth responses tend to occur in new, rapidly growing tissue found at the tips of plant shoots. These young cells possess the flexibility that allows them to grow quickly in the necessary direction, according to Professor von Sengbusch.