What Is Phototropism? - Definition, Experiments & Examples

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Sarah Friedl

Sarah has a Master's degree in Zoology and a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Why do plants bend toward sunlight? Phototropism is one way a plant can maximize its exposure to sunlight, and because plants need sunlight to make food, this is an important survival tactic.

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If you put a plant in a window you may notice that after a few days it has bent toward that window. This is because the plant needs sunlight to make energy, and so it grows toward the light. The growth of a plant toward any stimulus is called tropism, and the growth of a plant toward a light stimulus is called phototropism.

Phototropism in a plant that has been exposed to sunlight on only one side

Seedlings of plants grow straight upward in dark environments in order to reach the sunlight above ground. Once they break through the surface they start bending toward the light because the growth of cells on the dark side is faster than the cells on the light side. However, if the amount of light is the same on all sides of the plant then it will continue to grow straight upward instead of bending.

Early experiments

Early experiments of phototropism were based on the idea that plants were bending toward the warmth of the light, not necessarily the light itself. Charles Darwin performed many experiments in the late 1800s, and his results showed that photoreceptors (cells that detect light) in the tip of the plant communicated to the lower part of the plant that curves. From this he concluded that there must be some substance produced in the tip of the plant that is sent to the lower part of the plant, signaling it to bend.

Darwin's work eventually led to the discovery of a plant hormone called auxin. Auxin is the chemical signal that causes plants to elongate and grow cells faster on the side of the plant farthest from the light. When auxin collects in the cells on the side of the stem that is away from the light source, the cells on this side are stimulated to grow. This is what causes the curvature in the stem towards the light source.

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