How do seeds know which way to grow?

A sprouting seed does not need light to know which way is up. It senses the direction of gravity using specialised cells that contain tiny dense particles which settle to the bottom of the cell, telling the plant which way is down. This response, called gravitropism, makes the root grow downward toward water and anchorage, while the shoot grows upward toward the surface and light. Plant hormones, especially one called auxin, redistribute within the seedling to make one side grow faster than the other, bending it in the right direction. Once the shoot reaches light, it also begins to grow toward it.