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Question: What causes a green plant exposed to light, on only one side, to bend toward the source of light as ...

What causes a green plant exposed to light, on only one side, to bend toward the source of light as it grows ?

Explanation

Solution

The movement of plants towards the light is also known as phototropism. Biologically speaking it is a response to light which is directional and allows the plant to have a growth towards or away from the source of light. They can be of two types- positive phototropism, which is growth towards the light and negative phototropism, which is growth away from the light. Aerial parts of the plant show positive phototropism while the roots show negative phototropism.

Complete answer:
The scientists named Charles Darwin and Francis Darwin showed the movement of plants towards the source of light by their experiments. They analysed that the bending was not taking place when the tips of the stems were covered. They concluded that the light is perceived at the tips. They also stated that the signal is transferred downwards to the stem. The signal is mobile.
These signals are given through some chemical growth stimulants which moves to the shaded side from the lit side and causes growth towards the shaded side partially, which results in bending towards the light source. These chemicals cause cell elongation. These phototrophic substances absorb light in the blue range of spectrum. The scientists named this chemical, Auxin. This acts on specific photoreceptors and causes the desirable change. Auxin is transported towards the shady side. It is also transported towards the illuminated side but in less quantity. So, it is less active here. Light source is very important for this chemical to work. In the absence of a light source, it shows no bending.

Note:
This phenomenon of phototropism should not be confused with photoperiodism. Photoperiodism is a phenomenon which is related to the regulation of physiological functions of plants in response to the time period they are exposed to light. This acts on the flower and activates the reproduction of the plant. This explains why the plants reproduce at a certain time of the year.