Explainer: How a seed grows into a plant

If you’ve ever picked up an acorn from an oak tree or scooped out the inside of a pumpkin, you’ve held in your hand the makings of a giant tree or a whole pumpkin patch. Seeds come in all colors, shapes and sizes, from a tiny poppy seed to a giant coconut. Each one is a factory that has everything it needs to start making another plant.

Many plants produce flowers, which develop into fruits, beans or grains. The seeds of flowering plants are made up of three parts. There’s the baby plant, or embryo. Then there’s the endosperm. This is a packet of food the embryo will use as energy to grow until it has leaves and can make its own food. These two parts are encased inside a seed coat. This hard, protective layer keeps the embryo safe until it’s ready to sprout.

Seeds can sense when conditions are right to grow. There must be enough water and sunlight. The temperature must be just right, too. Then, the seed coat absorbs water, softens and cracks open. A tiny root called a radical grows down into the soil. The first shoot or stem — called a plumule — grows up toward the sunlight.

But how do roots and shoots know which direction to grow?

Like us, plants sense gravity. The cells of their roots and shoots contain small packets of starch granules. Those packets are heavy, so they settle at the bottom of cells. That tells a plant which way is down, explains Anna Newman-Griffis. She studied plant biology and now teaches at Durham University in England.

Root cells are programmed to grow down toward the pull of gravity. This is called positive gravitropism. Shoot cells have negative gravitropism. That tells them to grow upward, against the pull of gravity.

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