Why Do Plants Bend Towards Light?


Plants need sunlight to create energy. That energy, in turn, enables the plants to create the food or nutrition it needs to grow and produce flowers or bear fruit. Plants are a source of beauty, food and oxygen on earth. Whether plants grow wild or are carefully cultivated on farms, in gardens or even indoors, they all need the sun to varying degrees.


Photosynthesis is the process used by plants to convert light energy into chemical energy. Plants convert energy from the sun into oxygen and carbohydrates such as sugar, glucose and cellulose and starch, which provide nourishment and allow them to grow. The plants use carbon dioxide and water from our atmosphere along with the sun. The process of photosynthesis can only happen in the presence of sunlight.


It's not your imagination, plants do bend toward the sun. This is called phototropism and was first observed and explained by scientist Charles Darwin. The plant movement--tropisms--happens so slowly that you might not realize the plant is bending for several days. But plants do move toward light to capture the energy needed for photosynthesis. Darwin reported explained the phenomenon in his book, "The Power of Movement in Plants," in 1880. He conducted experiments using grass sheaths and concluded that plants moved toward the sun.

How it Works

Darwin found that plant cells on the side of the plant receiving little or no light eventually elongate. This allows the plant to bend in the direction of the light source. Since Darwin's time, other researchers have found that auxins, plant hormones also called indoleacetic acid, on the dark side of the plant are what cause the plant to bend. One of the more common examples of phototropic plants is the sunflower, which always follows the direction of sun. Marigolds, morning glories and day lilies also are natural sundials, because they follow the sun.

Other Effects

Plants detect sunlight through macromolecules called photo receptors. It is interesting to note that while the top or visible part of the plant moves toward the light, the roots of the plant grow away from sunlight.

Shade Plants

Though all plants need sunlight, some plants are considered shade plants because they do not tolerate full sun. The leaves of a shade plant are thinner, often wider or bigger, and have more chlorophyll than the leaves of sun-loving plants. As a result, shade plants can harvest sunlight more efficiently at low light levels. The leaves of sun plants, however, have a higher light saturation point and a "maximum rate of photosynthesis," according to John King, author of "Reaching for the Sun: How Plants Work."

Keywords: photosynthesis, bend toward sunlight, phototropism