Parts of a Plant

Almost every flowering plant on Earth is built from the same basic parts: roots to anchor it and draw up water, a stem to hold it up, leaves to capture sunlight, and (in flowering plants) flowers, fruits and seeds for reproduction. Each part has a very specific job, and they all work together so the plant can grow, feed itself and make the next generation.

  • Main parts6Roots, stem, leaves, flowers, fruits, seeds
  • Water transportXylemTubes carrying water up from the roots
  • Food transportPhloemTubes carrying sugars to the rest of the plant
  • Where food is madeLeavesIn chloroplasts via photosynthesis
  • Deepest tree rootsapprox. 70 metresShepherd's tree in the Kalahari
  • Biggest leavesRaffia palmUp to 25 metres long

Roots: the underground anchors

The roots do two key jobs: they hold the plant in place, and they suck up water and minerals from the soil. Roots branch out in all directions and end in tiny root hairs that give them a huge surface area for absorbing water. A mature wheat plant can have over 200 km of root hair packed into a few cubic centimetres of soil.

Different plants have different root shapes. Tap roots (like a carrot) have one big main root that goes deep. Fibrous roots (like grass) are a tangled network of similar-sized roots that spread out near the surface. Some desert plants have roots over 50 metres deep to reach buried water; others (like cacti) have wide shallow roots that suck up rain almost as soon as it falls.

Stem: the trunk and the plumbing

The stem holds the plant up off the ground, supports the leaves and flowers, and contains all the plant's internal plumbing. Inside every stem are two kinds of tube running from the roots to the leaves:

  • Xylem: carries water and dissolved minerals UP from the roots to the leaves.
  • Phloem: carries dissolved sugars made in the leaves DOWN to feed the rest of the plant, including the roots.

In a tree, the wood is mostly old layers of xylem. The newest growth is just under the bark. If you damage the bark all the way around a tree, you cut off the phloem too, the tree cannot send food down to its roots, and it will die. This is why old trees are sometimes attacked by gnawing animals.

Leaves: the solar panels

The leaves are where most of the plant's food is made. Leaves are usually flat and wide to catch as much sunlight as possible, and they contain millions of green chloroplasts where photosynthesis takes place. Inside each leaf is a network of tiny tubes (the leaf veins) that branch off from the xylem and phloem to carry water in and sugars out.

The underside of a leaf is covered in tiny pores called stomata that open and close to let in carbon dioxide and let out oxygen and water vapour. On a warm day, a single large tree can pump hundreds of litres of water out into the air through its stomata.

Flowers: making the next generation

The flowers are a plant's reproductive organs. Most flowers contain both male and female parts:

  • Stamens (male): produce pollen, a fine yellow dust that contains the plant equivalent of sperm.
  • Carpels (female): contain ovules, the plant equivalent of eggs.
  • Petals: often brightly coloured and scented to attract animals (mostly insects) to come and visit and accidentally carry pollen from flower to flower.
  • Sepals: small leaf-like flaps that protected the flower before it opened.

When pollen from one flower reaches the carpels of another flower of the same kind, the eggs are fertilised and start growing into seeds.

Fruits and seeds

After fertilisation, the flower's ovaries swell up and develop into a fruit, with one or more seeds inside. Some fruits are big and juicy (apples, peaches, strawberries) to encourage animals to eat them and accidentally carry the seeds away in their stomachs. Some fruits are hard or papery (acorns, peanuts, sycamore "helicopters") and rely on the wind, water, or burying animals to spread the seeds. Each seed contains a tiny embryo plant plus a packet of stored food to fuel its first few weeks of growth, all wrapped in a protective coat.

Fact A single mature oak tree can produce around 10,000 acorns in one good year, and over its 200-year life it might drop millions of acorns. Almost every single one is eaten by squirrels, jays, mice or deer. From all those millions, perhaps only one or two will eventually grow into a new mature oak. Most plants follow this same boom-and-bust strategy: produce huge numbers of seeds in the hope that one or two will get lucky.

How the parts work together

A plant is more than the sum of its parts. The roots draw up water; the xylem carries it up the stem; the leaves use it (and CO2 from the air and sunlight from the sky) to make food; the phloem carries that food back down to the roots and into developing flowers and fruit. Take away any one part and the whole plant suffers. Pull off the leaves and the plant cannot make food. Damage the roots and the plant dehydrates. Block the xylem and the upper parts wilt and die.

Did you know? The "strings" you see when you snap a stick of celery are bundles of xylem and phloem: the tubes that carry water and food up and down the plant. You can see them especially well if you stand a stalk of celery in water with red food colouring for a few hours: the dye is sucked up the xylem and stains the strings bright red all the way up to the leaves.
Deeper dive: how water gets to the top of a giant tree

The tallest trees on Earth are coast redwoods, with some over 115 metres tall. The very top branches need water, just like every other part of the tree. But how does water actually get all the way up there? It is a remarkable feat of physics.

A simple pump cannot push water that high (no pump on Earth can push water more than about 10 metres up before air bubbles form). So trees do not push water up; they pull it up. Every leaf has tiny stomata pores that constantly evaporate water into the air, a process called transpiration. The evaporating water at the top of the column literally pulls on the water below it, all the way down to the roots. Water molecules stick together extraordinarily well (a property called cohesion), so the whole column of water in the xylem is dragged up as one continuous chain.

If even one bubble of air gets into the xylem column, the whole pull breaks and the upper part of the tree cannot get water. This is why drought is so dangerous for tall trees: under stress, the water column can snap, and the tree may be unable to recover. Some giant trees may be close to the absolute maximum height that the laws of physics allow, around 130 metres.

For more, see photosynthesis and pollination. For how a plant grows from a seed, see plant life cycle.