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The Bends: Why Do Plants Move Towards Light?

  • Written by  Jolene Borrelli

All plants consume light to survive. Many will bend themselves outside of their natural growth habit to reach towards light. A plant on a windowsill will turn to face the sun, letting you know it’s not receiving enough solar rays. This phenomenon is known as phototropism. It demonstrates how necessary it is for plants to receive light and in the right amount.

Although plants may vary in their light requirements, all need to consume light to survive. A plant does not have a brain, nor does it have a nervous system. But it does have hormones that control the intricate workings of plant life.

In this case, the hormone auxin is responsible for helping plants lean towards light. In the tip of every shoot and root, as well as under woody fiber and bark, is the meristem tissue. This is the only place plant cells divide and thus the only place from which a plant truly grows. It is also where auxin is produced.

Auxin is photophobic. It flows from the meristem tissue to other parts of the plant, reacting to the presence of sunlight and moving away from it. If you see a plant growing directly upwards, its meristematic centers are receiving a balanced amount of light and its auxins are moving downward on all sides proportionately. In the absence of balanced light, auxins will soon distribute unevenly, away from the light, and accumulating on the shaded side of the plant.

Where the auxin accumulates, a plant’s cells will drop in pH and acidity. It is only in this acidic environment that hydrogen bonds are disrupted and the enzyme expansion can begin to break down the cell walls within the stem. As the cells weaken, they swell and elongate, causing the stem to bend.

When the tip of the stem and its meristematic tissue begin to receive balanced light once again, the stem will cease to bend and resume growing straight, remaining forever in a contorted position.

How a plant moves towards the sun is certainly curious. In one sense, it seems almost a conscious reaction that a plant would bend to move shaded parts of itself into light so that it can continue to photosynthesize at its full potential. At the same time however, it is natural for auxins to move towards the least amount of light, regardless.

Next time you see your indoor greenery leaning one way or the other, you can pin it on phototropism. A properly placed light or a nice spot to receive sun will prove a great help. Balanced light makes for happy and upward-growing plants!



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Last modified on Wednesday, 05 March 2014 09:50

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