We know, plant are living beings.They can respond.When a insect is being on plant, the plant can actually feel that insect.plants are able to hear sounds from evironment. They can sense the munching vibrations that the insects make. Therefore, we can say, whenever an insect is being on it, it can actually feel that insect.
Since it is roved by sir styendranath bose that plant can respond to the vibrations from the environment thus we can say that they know that there is a insect is on the branches or leaves by the vibration

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