Seeds

Open a bean and inside you will find a tiny plant, complete with the beginnings of a root and two little leaves, folded up and waiting. Around it is a store of food to get it started, and around that a tough coat. A seed is a plant in suspended animation, built to travel, to wait, and to begin growing at the right moment in the right place. It is also, more than anything else, what people eat.

What a seed is

A seed forms after a flower is fertilised. The egg becomes an embryo, a miniature plant with a root tip at one end and a shoot at the other. Beside or around it the parent plant stores food, starch, oil or protein, enough to feed the seedling until it can make its own. The whole package is sealed in a coat that keeps out water, fungi and insects. Then the seed dries, its metabolism slows to almost nothing, and it can remain alive in that state for months, years or, in some cases, centuries.

Seeds did not always exist. For hundreds of millions of years plants reproduced with spores, single cells that need damp conditions to grow. The seed, with its food store and waterproof coat, let plants colonise dry land, and the seed plants, conifers and then flowering plants, came to dominate every landscape.

Getting away

A seedling that sprouts beneath its parent will be shaded and starved, so seeds have evolved to travel. Dandelions and thistles hang theirs from parachutes; sycamores and maples give theirs spinning wings; poplars and willows wrap theirs in fluff. Burdock seeds are covered in hooks that catch in fur, the inspiration for Velcro. Coconuts float, and have crossed oceans to colonise islands thousands of kilometres from their origin. Many plants wrap their seeds in sweet fruit so that birds and mammals will eat them and drop them far away, with manure. Squirrels and jays bury acorns and forget some; ants carry seeds home for the oily parcels attached to them and discard the seed itself in their nutrient-rich rubbish heaps. A few plants, like gorse and some violets, fire their seeds from exploding pods.

Waiting

Many seeds will not germinate as soon as they land, even in good conditions. This dormancy stops them sprouting in autumn only to be killed by winter. Some need a period of cold before they wake; some need their coat scratched or softened by passage through an animal's gut; some need fire, which clears the competition, and germinate only after smoke or heat. Weed seeds can lie in soil for decades until ploughing brings them into the light.

The record for patience is remarkable. A date seed excavated from a 2,000-year-old fortress in Israel germinated in 2005 and is now a tree. Seeds of a 32,000-year-old Arctic flower, buried by a squirrel and frozen in permafrost, were coaxed into growing in a Russian laboratory.

Waking up

When a seed takes in water, it swells, the coat splits, and the embryo's cells begin to divide. The root pushes down first, to anchor the seedling and find water. Then the shoot pushes up, often bent over like a hook to protect the growing tip as it forces through the soil. In the dark it is pale; in the light it turns green and unfolds its first leaves. Until then it runs on the food the parent packed. A seedling that uses up its store before reaching the light dies.

The seeds we eat

Human civilisation is built on seeds. Wheat, rice and maize alone provide more than half of all the calories people eat, and barley, oats, millet, sorghum, beans, lentils, peas, soy, peanuts and nuts add much of the rest. All of them are seeds, selected over ten thousand years for size and the habit of staying on the plant to be harvested rather than scattering. Coffee, cocoa, pepper, mustard and nutmeg are seeds. Cooking oils are pressed from seeds. Bread, pasta, tofu, beer and chocolate are seeds, transformed.

Because the world's food depends on so few crops, their seeds are guarded. Seed banks around the world store hundreds of thousands of varieties, and the Svalbard Global Seed Vault, dug into a frozen mountain in the Arctic, holds backup copies of more than a million, against the day a disease, a war or a changing climate destroys a crop somewhere.

Why it matters

A seed is the simplest and most effective means ever devised for a living thing to send its descendants into an uncertain future. It is also the foundation of agriculture, and so of cities, writing, and everything that followed. The grain of wheat in a loaf of bread is the same device that let the first plants conquer dry land 350 million years ago.