Do Plants Grow Taller in Light or the Dark?

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A plant growing toward the sun.

For as long as botanists and gardeners have been engaged in the study of plants, they have noticed varied growth patterns under altered conditions of illumination. By regulating the amount of light that a plant receives, the height and rate of growth may be changed. Although it may seem counterintuitive, plants that are grown in darker conditions generally grow to be taller than plants grown in full light.


Plant Hormones

Also called plant growth regulators, plant hormones are synthesized in the cells of the stems, leaves, flowers and roots of plants. The hormone controlling the stem's ability to grow toward light is called auxin, after a Greek word meaning "to increase." Auxin's primary function is to stimulate increases in cell length near the root tips and stem, therefore elongating a plant. The more auxin that is located in a particular area of a stem, the longer that section of stem will become.


Symptoms of Low Light Conditions

Plants competing for sunlight or growing in dark conditions will develop elongated stems and poorly developed leaves. They are often tall and thin, lacking color in their stems. In severe cases, this physiological condition is called etiolation. Therefore, the darker the environment, the taller the plants tend to be.

Symptoms of Full Light Conditions

Short and thick stems, along with closely spaced leaves, result from full sun. If light strikes a plant from only one side, the stem will bend toward it. If a plant is illuminated from above, cells experience equal rates of elongation and the result is vertical growth. Generally speaking, the more light that a plant receives, the shorter it will be. It is not necessary for the plant to invest energy in growing vertically, since it receives all the light it requires.



Phototropism is the physiological process, caused by the plant hormone auxin, that occurs when stems grow toward the source of illumination. When light strikes one side of a stem, auxin builds up in the shady side, thereby causing the cells to grow faster and result in elongation on the shady side. Elongation on the shady side of the stem then results in the plant bending toward the light.