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.
There are two theories to explain this mechanism. It could be that the protoplasm (the living substance inside a cell) exerts greater pressure on the cell walls at the bottom of the seed. Or it may be that starch grains within the cells settle at the base. 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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