When a seed falls on the ground the plant doesn’t know which way up its going to fall. Out of one half grows the shoot and the other end grows the root but burying a seed upside down won’t make the roots grow into the air. The plant can work out which way is up in a phenomenon known as gravitropism. Auxin is a plant hormone that stimulates growth in the stem and root, but it stimulates it in different ways. The chemical itself is redirected by the plant due to gravity and so gathers in the lower half of the plant. The shoot grows where there is more auxin and so with the bottom half growing more the stem bends upwards. The root grows more where there is less auxin and so the upper half extends and the root bends downwards. Cells called statocytes containing particles called statoliths are generally accepted to be how the plant senses gravity.
It is however still a mystery how exactly the plants sense the presence of the statoliths inside the statocytes in order to direct the auxin to the specific locations and what details about the gravitational acceleration this provides to the plant. How come plants always grow upwards without fail when the wind and rain can produce an effective acceleration of up to three times the gravitational force in any direction. By using a centrifuge to generate an acceleration it was found that the plants can not detect the magnitude of the acceleration they are put in. In truth they aren’t even detecting the gravity at all. Instead they use their inclination regardless of gravity to determine what direction they should grow. This evidence has dismantled previous theories involving the pressure exerted by the statoliths in the plant being the primary method for gravitational detection. The experiment still needs to be repeated on roots but theories are already being created about the the colloidal nature of these particles and how they must move inside the satocytes in order to satisfy the observed affects.