Why Plants Bend Towards Light

                                                                                         

There are a number of plants that grow towards sun. This activity of plants, to bend towards light, is known as phototropism. Usually, the upper portion of the plant i.e. the stem and the leaves look for a sunny spot and try to reach out. For instance, sunflower follows the path of the sun across the sky. This characteristic of the plant enables its leaves to receive the light energy required for the process of photosynthesis. This is an extremely important process for the plants to grow and reproduce.

This bending toward light is called phototropism. Phototrophism is a response that causes house plants to lean towards the window and trees to branch over the road. Take a walk in the woods and look for fallen trees. Auxins cause fallen trees to turn at their tips and grow upright again.

The gene that causes plants to reach for the sky has been cloned. To the scientists who found it, the big surprise is that part of the protein made by the gene is duplicated in many other organisms, from simple bacteria to humans. And while that protein section, called a "domain," controls a plant's tendency to bend toward light, in other organisms it governs responses to different environmental factors, such as changes in oxygen levels.

You may have noticed that a houseplant grows toward the window and turns its leaves towards the light. It does this because light coming from the window side of the plant destroys the auxin in the shoots and leaves on that side, so that growth on this side slows down. On the shaded side of the plant there is more auxin, so growth on this side speeds up. The result is that the shoots and leaves are turned towards the light - maximising photosynthesis. This is an example of positive phototropism.

Many plants, too, move their flowers and their leaves so that they face the sun the greater part of the day. If you walk through a wood carpeted with wood-anemones on a sunny day, with your back to the sun, the ground seems covered with the little white flowers; but if you turn round and face the sun there are hardly any to be seen; for all the anemones are turning in the same direction, with their faces towards the sun, so they now have their backs towards you.

Plants can see the light, the very same way, which we do, by using specific macromolecules called as photoreceptors that can detect the light. In case of mammals rhodospin is the visual pigment of the eye. Plants too have similar type of the photoreceptor molecules to gather information about its light environment. Plants obtain information about its light environment by several ways using one of the properties of the light. The way plants elicit this information is illustrated in the table. To detect an unidirectional light the plants have to make a two point measurement to detect a light gradient within the plant body.

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