Plants need sunlight for photosynthesis, the process of producing food. Thus, when plants are kept away from the sunlight they bend or turn towards the light.
House plants kept near the window bend towards sunlight to gain as much light as possible. It has been observed that if house plants do not get sunlight they bend towards the artificial sunlight.
The movement of plant organs in response to incidence of rays of light is known as phototropism or heliotropism. Those organs which move towards the light are known to be positively heliotropic like for example the shoot of the plant. Those organs which grow away from light are said to be negatively heliotropic, for example the root.
The groundnut plant is a very interesting case in the study of heliotropism. The flower stalk is positively heliotropic and grows towards the light.
However, once the flower has been pollinated, the stalk becomes negatively heliotropic. More interestingly, it becomes geotropic, that is, it goes down into the soil. It pushes the fertilized ovary into the ground where it ripens and forms the pod.
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