Plants derive energy from the sun to make their own food, and therefore plant growth and light exposure are tightly interwoven. For example, plants such as the sunflower track the position of the sun as it crosses the sky. Houseplants grown near a window typically need to be turned to keep the plant from leaning toward the light. Light exposure also affects the rate of stem growth.
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The tight connection between light and stem growth is most starkly evident in the seedlings of certain types of plants. For example, among flowering plants, known as angiosperms, seedlings that grow in complete darkness develop highly elongated stems. This phenomenon is known as etiolation. Etiolated seedlings also show underdeveloped, yellow leaves that are held in a downward position.
Purpose of Etiolation
Etiolation derives from the normal process of germination. Most seeds first sprout in the total dark of their underground environment. Rapid stem elongation brings the plant's photosynthesising parts above the level of the soil. If there is also darkness above the soil, stem elongation continues just as if the seedling had not broken the soil surface.
A limited version of etiolation also occurs in adult plants that grow in insufficient light. Leaves do not grow to their customary size, but stems experience increased growth at least in terms of length, which results in "leggy" plants with a large internode distance. Nodes are placed along the stem where leaves or new branches can come off the stem; in light-deprived adult plants, increased stem growth places nodes further apart.
In seedlings, etiolation is mostly reversible. Exposing etiolated seedlings to light causes leaves to turn green and expand to normal size over the course of minutes. Etiolated seedling stems also cease growing longer and start putting on girth. In adult plants, however, leggy growth is permanent. Exposing the plant to increased light will prevent further legginess but will not reverse the existing changes.
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