Plants exhibit phototropism because
long roots plants absorb more water and nutrients
they need the sun's energy in order to produce food
water is essential to a plant's growth
strong stems are more wind resistant

The answer is they need the Sun's energy in order to produce food

Phototropism is plant growth in a response to light. In order to survive, plants need Sun's energy. During photosynthesis, carbon dioxide and water are converted into glucose and oxygen using the energy of sunlight. To get more light, plants usually grow in the direction of light, so more light could be absorbed and used in photosynthesis. Phototropism enables plants to absorb more light and that way use Sun's energy to produce food.

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