How do seeds know which way to grow? © Getty Images

How do seeds know which way to grow?

Plants need a different type of taxi to get their shoots up to the surface and their roots down in the dirt.

Asked by: Eileen Kennedy, Abingdon

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All plants can sense the direction of the gravitational field and orientate themselves accordingly. This is called geotaxis. In mature plants, phototaxis (growing towards the light source) overrides the gravitational impulse for the stalk and leaves, but the roots – and the seed while it is underground – rely on gravity for orientation.

The mechanism is thought to be based on either the protoplasm (the living substance inside a cell) exerting a greater pressure on the cell walls at the bottom, or starch grains within the cells settling at the bottom. One or both of these cues influence the production of plant growth hormones that cause the plant to ‘steer’ as it grows.

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