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, 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, and Ingen-Housz found that the effect happened only when the plant was in light. Ingen-Housz also showed that only the green parts of plants gave off oxygen, and the growth of plants goes with an increase in their carbon content.

In photosynthesis, a plant uses sunlight, water and carbon dioxide to make sugar, and it gives off oxygen. The oxygen released comes from splitting water, a step that makes no sugar. Plants can store the energy in glucose inside larger starch molecules. 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. Plants look green because of chlorophyll, which takes in blue and red light. 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.

A plant also has to sense where the light and the ground are. 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. Charles Darwin and his son Francis found in 1880 that a plant detects the light that makes it bend at its tip. Later, Went found that an uneven spread of auxin causes the bending toward light; the substance was named auxin.

A seed is a baby plant with a coat around it, and the embryo's root, called the radicle, develops into the root system of the plant. 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. A seed can wait, doing nothing, until the conditions are right. It also 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, apple seeds will not sprout unless they have a cold spell first, and some seeds germinate better in full light while others need darkness. Once germination has begun, a dry period will kill the embryo.

Once a plant is growing, it moves a great deal of water. Plants take up liquid water from the soil and let water vapor go from their leaves into the air. Root hairs extend from root cells and add surface area, so roots absorb water faster, and 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.

Many trees get help from fungi. 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. In dry summer months fog condenses on a redwood's leaves, and some of this moisture is absorbed directly.

Flowers are how plants make seeds. Some plants can pollinate themselves, and others need pollen carried from another flower; either way, pollen has to reach the flower's stigma to start seeds. More than 78 percent of flowering plants in temperate regions rely on insects and other animals for pollination. Cross-pollination is preferable to self-pollination because it produces more genetic diversity in plant populations. Petals help attract pollinators, and even coffee and chocolate would be less plentiful without pollinators.

Some seeds wait a remarkable time. 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; the researchers nicknamed the sapling Methuselah. Giant sequoia seeds are light and winged and are carried from the treetop by wind, and 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

• auxin: plant growth signal
• endosperm: seed food store
• transpiration: plant water loss
• phytoplankton: tiny ocean plants