You might have noticed that plants bend toward the light. This is an example of a tropism where light is the stimulus, known as phototropism. To obtain more light for photosynthesis, leaves and stems grow toward the light. On the other hand, roots grow away from light. This is beneficial for the roots, because they need to obtain water and nutrients from deep within the ground.

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Phototropism

So, how do the roots of seeds underground know to grow downward? How do the roots deep in the soil know which way is up? Gravitropism is a growth toward or away from the pull of gravity. Shoots, the new growth of a plant, also show a gravitropism, but in the opposite direction. If you place a plant on its side, the stem and new leaves will curve upward.

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Gravitropism

Plants also have a touch response called thigmotropism. If you have ever seen a morning glory or the tendrils of a pea plant twist around a pole, then you know that plants must be able to sense the pole. Thigmotropism works much like the other tropisms. The plant grows straight until it comes in contact with the pole. Then, the side of the stem that is in contact with the pole grows slower than the opposite side of the stem. This causes the stem to bend around the pole.

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Thigmotropism