What Is a Plant?

A plant is a living thing that makes its own food from sunlight, water and air. Almost every plant is green, almost every plant has roots, a stem and leaves, and almost every plant lives in one place its whole life. Plants are one of the six kingdoms of life on Earth, and they cover an astonishing range: from tiny duckweeds less than a millimetre wide, to giant redwood trees over 100 metres tall, to flowers with petals one centimetre across, to enormous old oaks that can live for 1,000 years.

• Known speciesapprox. 390,000And several thousand more found each year
• Four main groupsMosses, ferns, conifers, floweringIn order of evolutionary age
• TallestHyperion (redwood)116 m, in California
• Oldest individualapprox. 4,800 yearsA bristlecone pine in California
• First land plantsapprox. 470 million years agoSmall moss-like growth
• % of Earth's oxygenapprox. 50% from plantsOther 50% from ocean phytoplankton

What makes something a plant?

Plants are different from animals, fungi, bacteria and protists in several big ways.

• They make their own food from sunlight, water and carbon dioxide, in a process called photosynthesis.
• They are green, because they contain a pigment called chlorophyll in their cells.
• They cannot move from place to place (they are anchored by roots), although they can still move parts of themselves slowly.
• Their cells have a tough box-like cell wall made of cellulose, which gives plants their structure.

The four main types of plant

Botanists sort plants into four big groups, in the order they appeared on Earth.

• Mosses and liverworts: small, simple plants with no proper roots and no flowers. They were the first plants on land, about 470 million years ago, and still mostly live in damp shady places.
• Ferns: bigger plants with proper roots and stems, but still no flowers or seeds. Ferns reproduce by spores. They were the dominant plants of the Carboniferous swamps that became our coal.
• Conifers (gymnosperms): trees and shrubs that produce seeds in cones (like pines, firs and spruces). They appeared around 300 million years ago and dominated the age of the dinosaurs.
• Flowering plants (angiosperms): plants that produce flowers, fruit and seeds in a faster and more efficient way. They are the youngest group (around 140 million years old) but the most successful: around 90% of all plant species today are flowering plants.

Why we need plants

Almost everything on Earth depends on plants in one way or another.

• Oxygen: plants produce roughly half the oxygen we breathe (most of the rest comes from algae in the ocean).
• Food: every bite of food you eat is either a plant, or comes from an animal that ate plants.
• Climate: plants take carbon dioxide out of the air and lock it into wood, leaves and soil, slowing climate change.
• Habitat: forests, grasslands, jungles and wetlands are made of plants and shelter most other species on Earth.
• Medicine: around 25% of modern medicines were first discovered in plants.
• Materials: wood, cotton, rubber, paper, linen, sugar and many other everyday things all come from plants.

Fact The largest single living thing on Earth is a plant. Pando is a colony of quaking aspen trees in Utah, all genetically identical and all connected by a single huge root system. From above it looks like a forest of thousands of separate trees, but underground it is one organism. Pando covers 43 hectares, weighs around 6,000 tonnes, and is thought to be at least 14,000 years old, possibly much older.

How plants spread around the world

Plants live almost everywhere on land except the icy interior of Antarctica. They thrive in steamy tropical rainforests, on bare cold mountain tops, in the burning sand of the desert, and even floating on the ocean surface. To get to all these places, plants have evolved clever ways to spread their seeds: blown by the wind (like dandelions), carried by animals (like sticky burrs or tasty fruit), exploded out of pods (like impatiens or witch hazel), or even floated across oceans (like coconuts).

Did you know? The smallest flowering plant in the world is called Wolffia, a kind of tiny duckweed that floats on still water. A single Wolffia plant is less than 1 mm across and weighs around 0.1 milligrams. You could fit dozens onto the head of a pin. It still has roots, leaves, flowers and seeds, just unbelievably tiny ones.

Deeper dive: how plants conquered the land

For the first 3 billion years of life on Earth, almost nothing lived on land. The land was bare rock, harsh sunlight, no soil, no shelter and nothing to eat. All life lived in the seas.

The first plants to grow on land were small and moss-like, around 470 million years ago. They probably evolved from green algae that lived at the edges of shallow ponds. Early land plants faced enormous problems: how to stop drying out in the open air, how to support themselves without water to float in, how to absorb water from soil, and how to reproduce without water for sperm to swim through.

The earliest land plants (similar to today's mosses and liverworts) solved these problems only partially: they were short, stayed close to the ground and could only grow in damp habitats. The next big jump came around 400 million years ago with the appearance of vascular tissue: long tubes (xylem and phloem) that could carry water and nutrients up tall plant stems. This let plants grow much bigger. By 350 million years ago Earth had its first true forests, with tree-sized ferns and giant clubmosses up to 30 metres tall.

Once plants colonised the land they completely transformed it. They produced soil from broken-down rock, they pumped oxygen into the air, and they provided food and shelter for the first land animals. Most of the modern atmosphere and the modern land ecosystem traces back to that ancient green invasion of the continents.

For the details, see parts of a plant and photosynthesis. For the most successful kind, see flowering plants.