There are 3 main environmental influences; water, light and gravity. The effect of water is known as hydrotropism. This can be positive (the plant grows towards the stimulus in this case water) or negative (the plant grows away from the stimulus). The effect of light is called phototropism, this is also either be positive or negative. The plant hormone auxin is responsible for cells in the stem to grow more and for cells in the roots to grow less. The shaded side of the plant shoot contains more auxin as auxin is in a higher concentration on the shady side of the plant. This causes the cells furthest from the light to elongate and the stem grows towards the light. The shaded side of the root also contains more auxin, this causes these cells to grow slower and the roots grow away from the light. The effect of gravity is known as geotropism. This can either be positive (growth in the direction of gravity, usually roots growing downwards into the soil) or negative (growth in the opposite direction to gravity, usually the stem growing away from the ground. This is also caused by auxin. Auxin gathers on the side closest to the ground (follows gravity). This causes cells in the side closest to the ground in shoots to elongate quicker and grow away from gravity. In roots, the side closest to the ground grows slower and the root grows towards gravity.

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