How do seeds know to grow against gravity to reach the surface

When you plant a seed, or a root, or a bulb, they are buried underground and don't have any sense to feel gravity. So, how do they "know" where the surface is?

Actually, it has nothing to do with heat or light, because plants grown in zero gravity will grow in all directions, yet seedlings grown in total darkness will still put roots down and stems up. Seeds do have a specific sense of gravity, and it is called gravitropism, or geotropism. Plants have a hormone called auxin that responds to the Earth's gravitational pull, and this causes roots to grow down and stems to grow up.

Basically, due to molecular processes, auxin is in greater concentration on the lower side of the cell membrane than the upper side. Among many other functions, auxin controls the rate of cell expansion, but the particular way that each cell reacts to auxin levels depends on the kind of cell it is. Root cells expand less in the presence of auxin so a horizontal root shoot will grow faster on its top surface than its bottom, which bends it downwards. The cells in the stem react the opposite way, so that shoot bends upwards.

Warmth and sunshine attract it to the surface.