A seed packs a whole plant into a very small space. Inside the coat is a baby plant, and the embryo's root, called the radicle, develops into the root system of the plant. Some seeds store their food in endosperm, a starch-rich tissue around the embryo. Plants with two seed leaves are dicots, while grasses can be monocots, with one seed leaf. A dry seed typically holds less than 10% water before it begins to take in water. The coat protects the baby plant from injury and from drying out.

A seed can wait, doing nothing, until the conditions are right. Then a dry seed soaks up water and swells, and it needs air with oxygen in it, because 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, and apple seeds will not sprout unless they have a cold spell first. Some seeds germinate better in full light, and others need darkness to germinate. Once germination has begun, a dry period will kill the embryo.

When a seed sprouts, its first root comes out of the seed coat first. After the root, a shoot grows up with the leaves and the stem. A tropism is a growth movement whose direction is set by the direction the stimulus comes from, and roots that grow down and away from light are more likely to find the soil, water and minerals they need. Roots hold a plant in the ground and take in water. Root hairs extend from root cells and add surface area, so roots absorb water faster.

Once a plant is up, water flows through it. Plants take up liquid water from the soil and let water vapor go from their leaves into the air. Leaves have a network of veins that carries water and nutrients within the leaf. 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. During a growing season, a leaf will transpire many times more water than its own weight.

In photosynthesis, a plant uses sunlight, water and carbon dioxide to make sugar, and it gives off oxygen. A stem holds the leaves up so they can reach the sunlight. The oxygen released in photosynthesis comes from splitting water, a step that makes no sugar. Plants look green because of chlorophyll, which takes in blue and red light. Plants are not the only makers of oxygen: 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.

Where does a plant's weight come from? In the 1600s van Helmont planted a 5-pound willow shoot in soil he had moistened with rain water. He kept the plant 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.

Many trees have a partner under the ground. 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. In dry summer months fog condenses on a redwood's leaves, and some of this moisture is absorbed directly.

How long can a seed wait? 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. Giant sequoia seeds wait for a different signal: 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.

Words to know

• endosperm: seed food store
• tropism: growth movement
• transpire: lose water vapor
• germination: a seed sprouting