In the 1600s van Helmont planted a 5-pound willow shoot in soil he had moistened with rain water, and for five years he kept it watered with rain water or distilled water. He theorized that the willow's weight gain came from water alone; he did not consider that gases in the air might be involved. Then, in 1772, Joseph Priestley reported the first evidence that gases take part in photosynthesis. Ingen-Housz found that the effect Priestley saw happened only when the plant was in light, and that a plant kept in the dark in a sealed chamber uses up oxygen, just as a mouse or a candle does.

Today we can say more. 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. On the space station, Veggie's red, blue and green LED lights grow plants; its plant pillows held 'Outredgeous' red romaine lettuce seeds, and each got about 100 milliliters of water.

When a seed sprouts, its first root comes out of the seed coat first, and 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. 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. Root hairs extend from root cells and add surface area, so roots absorb water faster.

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.

Plants rely on wind, water, animals and even people to spread their seeds. Seeds that travel by wind are small and light, with a special shape such as small hairs or paper-thin wings. Giant sequoia seeds are light and winged and are carried from the treetop by wind, and most land near the parent tree, though windstorms can carry them as far as 400 meters from tall trees. Animals can carry undigested seeds inside them and drop them somewhere else. Common burdock seeds stick to fur like Velcro. A coconut is considered one of the largest seeds in the world.

A seed can wait a very long 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 of the ancient palm appeared, and the researchers nicknamed it Methuselah. Giant sequoia seeds wait for fire: they must fall on mineral soil left bare by fire to sprout and survive, and when fire burns through a sequoia forest, hot air dries the old cones.

Plants come in extreme sizes and places. 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.

Words to know

• stomata: tiny leaf openings
• tropism: growth movement
• pollinators: pollen carriers
• chlorophyll: green plant stuff