A seed is a small bundle of stored resources; a seed is a baby plant with a coat around it, and the coat protects the baby plant from injury and from drying out. A dry seed typically holds less than 10% water before it begins to take in water, and some seeds store their food in endosperm, a starch-rich tissue around the embryo. The embryo's root, called the radicle, develops into the root system of the plant, while plants with two seed leaves are dicots and grasses can be monocots, with one seed leaf. Seeds contain the genetic information to produce a new plant. A seed needs water to start to grow, and it needs to be warm enough to start to grow, but the seed's coat can be thin and soft, like a bean's, or very hard, like a coconut's.

A seed does not spend its savings until the conditions are right; it can wait, doing nothing, and then a dry seed soaks up water and swells. It needs air with oxygen in it, and soil soaked with water can leave a seed with too little oxygen. Most vegetable seeds sprout best in soil that is about 65 to 75 degrees Fahrenheit, apple seeds will not sprout unless they have a cold spell first, and some seeds germinate better in full light while others need darkness to germinate. Once germination has begun, a dry period will kill the embryo, so the timing of that first drink matters a great deal. Seeds are usually planted only about two or three times as deep as they are wide, and a hard coat may need to be scratched before the seed will sprout.

The most patient seeds show how long the savings can last; a date palm seed about 2,000 years old, found at Masada, grew into a young tree. The ancient date seeds had sat on a shelf for decades before botanists soaked them in hot water and nutrients and planted three of them in enriched soil in 2005. Three months after planting, the soil cracked and a single shoot appeared, and the researchers nicknamed the sapling Methuselah. Giant sequoia seeds wait for a different signal, because they must fall on mineral soil left bare by fire to sprout and survive. When fire burns through a sequoia forest, hot air dries the old cones, and within one to two weeks they rain down seeds on the bare soil; the seeds are light and winged, and windstorms can carry them as far as 400 meters from tall trees.

Once the first leaves open, the plant earns its income; in photosynthesis a plant uses sunlight, water and carbon dioxide to make sugar, and it gives off oxygen. The oxygen released in photosynthesis comes from splitting water, a step that makes no sugar, and plants can store the energy in glucose inside larger starch molecules. Plants look green because of chlorophyll, which takes in blue and red light; leaves take in carbon dioxide and give off oxygen through tiny openings called stomata, and on a hot, dry day plants close their stomata to conserve water. As of November 2026, marine phytoplankton produce over half of the oxygen on Earth, and they are photosynthetic organisms at the base of ocean food webs.

The price of that income is water; plants take up liquid water from the soil and let water vapor go from their leaves into the air, and root hairs, which extend from root cells, add surface area so that roots absorb water faster. Transpiration, rather than root pressure, is typically the force that moves water up in a plant. A large oak tree can transpire 40,000 gallons (151,000 liters) of water per year, an acre of corn gives off about 3,000 to 4,000 gallons (11,400 to 15,100 liters) of water each day, and during a growing season a leaf will transpire many times more water than its own weight. In dry summer months fog condenses on a redwood's leaves, and some of this moisture is absorbed directly.

Some of the profit is paid out underground; trees give fungi sugars in the form of carbon from photosynthesis, and the fungi deliver nitrogen, phosphorus and minerals and help the tree take up water. In a redwood forest, the fungi weave their threads into a mat of roots and make a vast network. Redwoods have shallow roots, only 6 to 12 feet down, but one redwood's roots can spread 60 to 100 feet around it. The trade is not always even, though; some species may draw more than they give.

It took centuries to understand the books; in the 1600s van Helmont planted a 5-pound willow shoot in soil he had moistened with rain water, and he kept it watered with rain water or distilled water for five years. He theorized that the willow's weight gain came from water alone. He did not consider that gases in the air might be involved. Joseph Priestley reported in 1772 the first evidence that gases take part in photosynthesis. Ingen-Housz found that the effect happened only when the plant was in light, and that only the green parts of plants gave off oxygen. The growth of plants goes with an increase in their carbon content.

Plants also steer; roots that grow down and away from light are more likely to find the soil, water and minerals they need. A tropism is a growth movement whose direction is set by the direction the stimulus comes from. Charles Darwin and his son Francis found in 1880 that a plant detects the light that makes it bend at its tip, and Went found that an uneven spread of auxin causes the bending toward light. Gravitropism, the way roots grow down, also involves an uneven spread of auxin.

Words to know

• endosperm: seed food store
• transpiration: plant water loss
• auxin: plant growth signal
• germination: a seed sprouting