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. Went later found that an uneven spread of auxin causes the bending toward light; the substance was named auxin. Gravitropism, the way roots grow down, also involves an uneven spread of auxin.

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. A plant kept in the dark in a sealed chamber uses up oxygen, just as a mouse or a candle does. 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.

The modern picture is that 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, and a stem holds the leaves up so they can reach the sunlight. 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 are not alone in this work: 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.

Plants also move a great deal of water. They 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, and during a growing season a leaf will transpire many times more water than its own weight.

Under the ground, many trees trade with 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, and redwoods have shallow roots, only 6 to 12 feet down, though one redwood's roots can spread 60 to 100 feet around it. The trade is not always even, because some species may draw more than they give. Above ground, redwoods have another source of water: in dry summer months fog condenses on a redwood's leaves, and some of this moisture is absorbed directly.

A seed is a baby plant with a coat around it, and it can wait, doing nothing, until the conditions are right. 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. 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. Once germination has begun, a dry period will kill the embryo.

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, and three months later a single shoot appeared; the researchers nicknamed the sapling Methuselah. Giant sequoia seeds 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. The seeds are light and winged, and windstorms can carry them as far as 400 meters from tall trees.

Trees grow to extremes. Coast redwoods are the tallest trees on Earth, about 380 feet tall, and as of November 2026 the tallest one known is 379 feet. As of November 2026, the General Sherman Tree is the world's largest tree, measured by volume. Some coast redwoods can live up to 2,000 years, and bristlecone pines, the oldest, can reach 5,000 years. Flowers are how plants make seeds. More than 78 percent of flowering plants in temperate regions rely on insects and other animals for pollination, and cross-pollination is preferable to self-pollination because it produces more genetic diversity in plant populations.

Words to know

• gravitropism: growing with gravity
• transpire: lose water vapor
• endosperm: seed food store
• stomata: tiny leaf openings