A seed is a baby plant with a coat around it; the coat protects the baby plant from injury and from drying out; inside, 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; grasses can be monocots, with one seed leaf. A dry seed typically holds less than 10% water before it begins to take in 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 giant sequoia starts as a seed about the size of a grain of oatmeal.
A seed does not sprout the moment it is dropped; it can wait, doing nothing, until the conditions are right; a seed needs water to start to grow, and it needs to be warm enough; then a dry seed soaks up water and swells. It also 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; some seeds germinate better in full light, and others need darkness to germinate; once germination has begun, a dry period will kill the embryo.
Some seeds wait far longer than a season; 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 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. The 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.
A seed that wakes up is still in the place where it fell, so the parent plant has to find ways to send its seeds out; 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, like maple and dandelion seeds; burrs have little hooks that stick to animal fur, and common burdock seeds stick to fur like Velcro. Animals can also carry undigested seeds inside them and drop them somewhere else; a coconut is considered one of the largest seeds in the world, and it is a seed that floats. It can drift across the ocean and grow in a new place, and drifting is more common for seeds in tropical, coastal places.
Flowers are how plants make seeds; pollination is moving pollen from the anther of a flower to the stigma; 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; after pollination, the ovule inside a flower becomes the seed and the ovary becomes the fruit.
Below ground, a growing plant has to find water and minerals; 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; many trees also 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. 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, because some species may draw more than they give.
Water then has to travel upward, and the main force is typically transpiration; plants take up liquid water from the soil and let water vapor go from their leaves into the air; transpiration, rather than root pressure, is typically the force that moves water up in a plant; leaves have a network of veins that carries water and nutrients within the leaf. A large oak tree can transpire 40,000 gallons (151,000 liters) of water per year; during a growing season, a leaf will transpire many times more water than its own weight. On a hot, dry day, plants close their stomata to conserve water; in dry summer months fog condenses on a redwood's leaves, and some of this moisture is absorbed directly.
The leaf is where light becomes food; 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 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 water vapor leaves through them too; 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.
It took centuries of experiments to understand this; in the 1600s van Helmont planted a 5-pound willow shoot in soil he had moistened with rain water; 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, and 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 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. Ingen-Housz also showed that only the green parts of plants gave off oxygen. The growth of plants goes with an increase in their carbon content.
A plant also steers itself; 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. Went found that an uneven spread of auxin causes the bending toward light, and the substance was named auxin. Gravitropism, the way roots grow down, also involves an uneven spread of auxin. The long results are visible in trees: a tree's yearly growth changes with the seasons, so its rings record moisture, temperature and sunlight; coast redwoods are the tallest trees on Earth, about 380 feet tall, and as of November 2026 the tallest one known is 379 feet. Some coast redwoods can live up to 2,000 years, and bristlecone pines, the oldest, can reach 5,000 years.
Words to know
- endosperm: seed food store
- transpiration: plant water loss
- gravitropism: growing with gravity
- phytoplankton: tiny ocean plants
