Seeds started indoors with inadequate light, undergo "etiolation," a phenomenon during which plant stems grow tall and spindly and fail to develop green colouration. If etiolation is allowed to progress, eventually the plants will fall over and die. Plants have evolved etiolation as a survival mechanism that allows them to reach a light source and begin the process of photosynthesis, during which they produce sugar for themselves from light energy.
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Plants require light in order to live. Unlike animals and many other organisms, they do not consume food; instead, they produce their own food energy using light. When a plant emerges from its seed, it usually emerges in the dark, under a layer of soil. The seed contains an energy-rich substance called endosperm that powers the plant's first days of growth, until it can reach the light and begin producing its own energy. In its first days of growth, a plant needs to reach a light source before it depletes the endosperm in its seed.
Although the root emerges first from the seed, anchoring it in place, the stem emerges next, and once it does, the plant pours the majority of its energy into stem growth. During its early days of growth, stems elongate quickly, reaching for the soil surface and a light source. If, upon emerging, the plant still doesn't receive adequate light, it continues this rapid elongation in an attempt to find a light source.
Mature Plants and Etiolation
Mature plants shaded by other plants will also display etiolation as they attempt to grow out of the shade and access a stronger light source. Unlike etiolation observed in seedlings, which is caused by inadequate light, specific wavelengths of light cause this phenomenon in mature plants. When grown beneath other plants, an abundance of far-red wavelengths of light reach the plant and provoke the production of a growth hormone that causes the stems to elongate. Far-red wavelengths of light also cause the plant to flower sooner. Both the elongation and the flowering are survival mechanisms that help the plant to reach the light it needs to survive and allow it to reproduce if it does not.
Preventing and Fixing Etiolation
When starting seedlings indoors, providing adequate light will prevent etiolation and encourage compact, healthy growth. Once seeds germinate, move them to a sunny south-facing window, if possible, or provide artificial lighting, using two 40-watt, cool-white fluorescent tubes. Place them 6 inches above the seedlings and use the lights for 16 hours per day, if they plants aren't receiving any sunlight.
You cannot undo the spindly growth produced because of etiolation. However, moving the plant to a location where it receives adequate light will stop further etiolation and allow the seedling to survive and continue healthy growth.
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