Plants have specialised gravity sensing cells at the root tip that contain ‘amyloplasts’. Amyloplasts are starch-filled structures that are heavier than the other things in the plant cell, so they sink to the bottom. Therefore, plants know that the direction amyloplasts sink is ‘down’ and the opposite way is ‘up’. Plant use this gravity recognition process as a guide to spread the plant growth hormone auxin unevenly inside plant organs. This directs roots towards the ground and shoots towards the sky.
That’s why plant roots always grow ‘down’, and shoots grow ‘up’. Lights and water also influence plants directional growth.