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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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Definition

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
phototropism

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.

Examples

There are two types of phototropism: positive and negative. Positive phototropism is growth toward a light source, and negative phototropism is growth away from a light source. Plant roots tend to exhibit negative phototropism, because they need to grow deeper into the ground for plant stability, water, and nutrients. Plant roots also exhibit gravitropism, which is growth in response to gravity.

Sunflowers are a great example of positive phototropism, because not only do their stems curve toward light but their flowers turn to face the sunlight as well. As mentioned above, your houseplants also likely exhibit phototropism. If you notice them bending toward the window, you can simply turn them around to help them grow straight again!

Summary

Most plants exhibit phototropism because it directs them toward sunlight, which they need to survive. A plant can do this because of photoreceptors in the tip of the plant that signal hormone production on the dark side of the stem. This creates an unequal amount of growth on one side of the plant, which then bends the plant toward the light, its ultimate food source.

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