Everything two big, heavy science books say about how trees work, told so a five-year-old can follow it. 121 questions, 19 diagrams to play with.
Tap a part of the tree to jump to it. In the answers, hover over or tap any underlined words to see their grown-up word.
water going up sugar going down
Up in the sky
A tree is a tall, long-lived plant built from billions of tiny boxes called cells, and it only grows at a few special spots.
A tree is a plant that decided to grow really, really tall and live for a really, really long time. To do that it builds a hard wooden body that stays standing all winter, instead of starting again from the ground every year like a daisy does.
Grown-up words: woody perennial is the grown-up word for ‘live for a really, really long time’ secondary growth is the grown-up word for ‘builds a hard wooden body’
Some trees keep their seeds in cones, like pines and spruces. Others grow flowers and keep their seeds in fruits, like apples and oaks. The cone trees are the older family. They were around long before there were any flowers at all.
Grown-up words: gymnosperms (conifers) is the grown-up word for ‘keep their seeds in cones’ angiosperms (flowering plants) is the grown-up word for ‘grow flowers and keep their seeds in fruits’ broadleaves is the grown-up word for ‘like apples and oaks’
Every bit of a tree is made of tiny boxes called cells, far too small to see. Each box has a stiff wall round it and is glued to the boxes next to it, with tiny tunnels joining them up. So, unlike yours, tree cells can never walk about. Where a cell is born is where it stays.
Grown-up words: cells is the grown-up word for ‘tiny boxes called cells’ cell wall is the grown-up word for ‘a stiff wall round it’ middle lamella is the grown-up word for ‘glued to the boxes next to it’ plasmodesmata is the grown-up word for ‘tiny tunnels joining them up’
Tap a part of the cell to find out what it does.
Grown-up words: the wall is the cell wall, the water bag is the vacuole, the instruction centre is the nucleus, the solar panels are chloroplasts, and the power stations are mitochondria.
Cell walls are made of strong, stringy threads. Some walls also get an extra, thicker layer soaked in a tough, woody glue. That glue is what lets a tree stand up tall. Plants without it, like moss, can only grow a few centimetres off the ground.
Grown-up words: cellulose is the grown-up word for ‘strong, stringy threads’ secondary cell wall is the grown-up word for ‘an extra, thicker layer’ lignin is the grown-up word for ‘a tough, woody glue’
Inside every cell are tiny machines that build things, break things and pass energy about in tiny rechargeable batteries, which little power stations keep charged. They work faster when it’s warm and slower when it’s cold, which is one reason trees are sleepy in winter.
Grown-up words: organelles is the grown-up word for ‘tiny machines’ enzymes is the grown-up word for ‘build things, break things’ adenosine triphosphate (ATP) is the grown-up word for ‘tiny rechargeable batteries’ mitochondria is the grown-up word for ‘little power stations’
A tree doesn’t grow everywhere at once. It only grows at a few special spots: the very tips of shoots, the very tips of roots, and one thin layer wrapped all round the trunk and branches. Those spots are full of baby cells that keep splitting in two. The tips make the tree longer, and the thin layer makes it fatter.
Grown-up words: meristems is the grown-up word for ‘a few special spots’ apical meristems is the grown-up word for ‘the very tips of shoots, the very tips of roots’ vascular cambium is the grown-up word for ‘one thin layer wrapped all round the trunk and branches’ primary growth is the grown-up word for ‘make the tree longer’ secondary growth is the grown-up word for ‘makes it fatter’
Grown-up words: the growing tips are apical meristems, and making the tree longer is primary growth. The ring-making layer is the vascular cambium, and making the trunk fatter is secondary growth.
In the leaves
Leaves catch sunlight and use it to turn air and water into sugar. That sugar is the tree’s food.
Leaves catch sunlight and use its energy to stick together a gas from the air and water from the soil to make sugar. That sugar is the tree’s food. The leftover bit is oxygen, which floats out of the leaf, and that’s the air you breathe.
Grown-up words: photosynthesis is the grown-up word for ‘catch sunlight and use its energy’ carbon dioxide (CO₂) is the grown-up word for ‘a gas from the air’ glucose is the grown-up word for ‘make sugar’ oxygen (O₂) is the grown-up word for ‘The leftover bit is oxygen’
Grown-up words: this is a light response curve. The dim point where the leaf makes exactly as much sugar as it burns is the light compensation point. The point where more sun stops helping is light saturation.
Leaves are packed with tiny green bags full of a green colour that is brilliant at catching red and blue light. Green light isn’t much use to it, so it bounces off, and that bounced green light is what lands in your eyes.
Grown-up words: chloroplasts is the grown-up word for ‘tiny green bags’ chlorophyll is the grown-up word for ‘a green colour’ absorption spectrum is the grown-up word for ‘catching red and blue light’
Step one: the leaf uses light to split water apart and charge up lots of tiny batteries. Step two: using those batteries, it grabs bits of air one at a time and clicks them together into sugar, a bit like building with Lego.
Grown-up words: light reactions is the grown-up word for ‘the leaf uses light to split water apart’ ATP and nicotinamide adenine dinucleotide phosphate (NADPH) is the grown-up word for ‘lots of tiny batteries’ Calvin cycle is the grown-up word for ‘grabs bits of air one at a time and clicks them together into sugar’
The helper that grabs air out of the leaf is called Rubisco, and it’s a bit clumsy. Sometimes it grabs oxygen by mistake, which wastes energy, especially on hot days. Some plants, like maize, have clever tricks to stop that. Almost all trees just use the ordinary way.
Grown-up words: Rubisco is the grown-up word for ‘The helper that grabs air out of the leaf’ photorespiration is the grown-up word for ‘grabs oxygen by mistake’ C₄ photosynthesis is the grown-up word for ‘clever tricks to stop that’ C₃ photosynthesis is the grown-up word for ‘the ordinary way’
Under each leaf are thousands of tiny mouths. They open to let air in for making food, but every time they open, some water escapes out. So a tree is always choosing: eat, or save water?
Grown-up words: stomata is the grown-up word for ‘thousands of tiny mouths’ gas exchange is the grown-up word for ‘let air in for making food’ transpiration is the grown-up word for ‘some water escapes out’
Open: air gets in so the leaf can make food, but water vapour escapes out of the same hole.
Grown-up words: each tiny mouth is a stoma (more than one are stomata), and the two beans are guard cells. On hot, dry days a message called abscisic acid (ABA) tells them to shut.
Just like you can get sunburnt, leaves can get more light than they can use. They have ways to throw the spare energy away as a tiny bit of heat, so it doesn’t break anything inside.
Grown-up words: photoinhibition is the grown-up word for ‘more light than they can use’ non-photochemical quenching is the grown-up word for ‘throw the spare energy away as a tiny bit of heat’
Leaves at the sunny top of a tree are small and thick. Leaves down in the shade inside the tree are big and thin, so they can catch every scrap of light. Same tree, two kinds of leaf!
Grown-up words: sun leaves is the grown-up word for ‘small and thick’ shade leaves is the grown-up word for ‘big and thin’
Pine and spruce needles are leaves, just thin, waxy and tough. That makes them hard to dry out and not very tasty to eat. Flat leaves catch light better but aren’t as tough.
Grown-up words: gymnosperm leaves is the grown-up word for ‘Pine and spruce needles’ cuticle is the grown-up word for ‘waxy’ sclerophyllous leaves is the grown-up word for ‘tough’
Some leaves, like ash and rowan leaves, are split into lots of small leaflets on one stalk. The whole thing counts as one leaf, and it drops off all in one go in autumn.
Grown-up words: pinnate compound leaf is the grown-up word for ‘split into lots of small leaflets’ rachis is the grown-up word for ‘one stalk’
Young trees sometimes grow leaves that look nothing like their grown-up leaves. Eucalyptus trees are famous for it: round baby leaves, long thin grown-up ones.
Grown-up words: juvenile foliage is the grown-up word for ‘leaves that look nothing like their grown-up leaves’ heteroblasty is the grown-up word for ‘round baby leaves, long thin grown-up ones’
Evergreen trees keep their leaves through winter and swap old ones for new a few at a time. Deciduous trees drop all their leaves in autumn and grow a fresh set in spring. Both ways work. It depends where the tree lives.
Grown-up words: evergreen is the grown-up word for ‘keep their leaves through winter’ leaf lifespan is the grown-up word for ‘swap old ones for new a few at a time’ deciduous is the grown-up word for ‘drop all their leaves in autumn’
Before dropping its leaves, a tree pulls the useful stuff out of them and saves it in its branches. Once the green has gone, yellows and oranges that were hiding underneath show through. Some trees make brand-new reds too. Then a little seal grows across the stalk and the leaf lets go.
Grown-up words: senescence is the grown-up word for ‘Before dropping its leaves’ nutrient resorption is the grown-up word for ‘pulls the useful stuff out of them’ carotenoids is the grown-up word for ‘yellows and oranges’ anthocyanins is the grown-up word for ‘brand-new reds’ abscission zone is the grown-up word for ‘a little seal grows across the stalk’
Still in the leaves
It feels how much light there is, what colour the light is and how long the nights are, and it keeps a clock inside.
Trees have special light-feelers inside them. One kind feels red light and can tell when another plant is shading it, because leaves above soak up red. Other kinds feel blue light.
Grown-up words: photoreceptors is the grown-up word for ‘special light-feelers’ phytochrome is the grown-up word for ‘One kind feels red light’ red to far-red ratio is the grown-up word for ‘leaves above soak up red’ cryptochromes and phototropins is the grown-up word for ‘Other kinds feel blue light’
Put a plant on a windowsill and it leans towards the window. The shady side of the stem grows faster than the sunny side, so the stem bends over towards the light.
Grown-up words: phototropism is the grown-up word for ‘leans towards the window’ auxin redistribution is the grown-up word for ‘The shady side of the stem grows faster’
Trees can tell how long the nights are. When nights get long in autumn, they know winter is on its way and start getting ready: they stop growing and make tough winter buds.
Grown-up words: photoperiod is the grown-up word for ‘how long the nights are’ short-day response is the grown-up word for ‘When nights get long in autumn’ bud set is the grown-up word for ‘make tough winter buds’
Grown-up words: the length of day and night is the photoperiod, the yearly timetable is phenology, making winter buds is bud set, counting cold days is the chilling requirement, buds opening is budburst, and leaves ageing in autumn is senescence.
Trees have a clock inside them that goes round once a day, even if you keep them in the dark. It helps them get ready for sunrise before the sun is even up.
Grown-up words: circadian rhythm is the grown-up word for ‘a clock inside them that goes round once a day’
In winter, buds fall into a deep sleep. One warm day won’t wake them. They need to feel lots of cold first, so they don’t get tricked into opening too early and getting frozen.
Grown-up words: dormancy is the grown-up word for ‘a deep sleep’ chilling requirement is the grown-up word for ‘feel lots of cold first’ premature budburst is the grown-up word for ‘opening too early’
Trees wait until they’re old enough before they make flowers, which can take many years. Then they pick the right time of year using day length and temperature.
Grown-up words: juvenile phase is the grown-up word for ‘wait until they’re old enough’ floral induction is the grown-up word for ‘pick the right time of year’
All over the tree
It uses tiny amounts of special chemicals that travel from one part to another and say grow, stop, sleep or wake up.
Different parts of a tree send each other messages using special chemicals. A tiny bit travels to another part and tells it what to do: grow, stop, sleep, wake up.
Grown-up words: plant hormones is the grown-up word for ‘special chemicals’ signal transduction is the grown-up word for ‘tells it what to do’
The tip at the top of a shoot sends a message downwards saying, ‘I’m in charge, you lot wait.’ That’s why buds lower down stay asleep, and why lots of trees grow one tall main stem. Cut the tip off and the buds below wake up.
Grown-up words: auxin (indole-3-acetic acid, IAA) is the grown-up word for ‘sends a message downwards’ apical dominance is the grown-up word for ‘buds lower down stay asleep’
The top bud sends a message down the stem. The buds below get the message and stay asleep.
Grown-up words: the ‘boss’ message is auxin, the top bud holding the others back is apical dominance, and the side buds waking up is lateral bud release.
Another message tells stems to stretch out longer. It also helps seeds wake up and start growing.
Grown-up words: gibberellins is the grown-up word for ‘Another message’ stem elongation is the grown-up word for ‘stretch out longer’ germination is the grown-up word for ‘seeds wake up’
Roots send a message up to the shoots saying, ‘Split, make more cells, and wake those buds up.’ The top and bottom of a tree are always chatting, so they grow in balance.
Grown-up words: cytokinins is the grown-up word for ‘Roots send a message up to the shoots’ cell division is the grown-up word for ‘make more cells’
One message is actually a gas. It helps fruit ripen and leaves fall off, and it tells the tree when it has been hurt or squashed. Put one ripe apple in a bag with some green ones and they ripen faster!
Grown-up words: ethylene is the grown-up word for ‘One message is actually a gas’ fruit ripening is the grown-up word for ‘helps fruit ripen’ abscission is the grown-up word for ‘leaves fall off’ wound response is the grown-up word for ‘hurt or squashed’
When roots feel the soil drying out, an alarm message rushes up to the leaves: ‘Shut your mouths, save water!’ The same message helps buds and seeds go to sleep for winter.
Grown-up words: abscisic acid (ABA) is the grown-up word for ‘an alarm message’ stomatal closure is the grown-up word for ‘Shut your mouths, save water!’ dormancy is the grown-up word for ‘go to sleep for winter’
Out on the branches
Each branch reaches for light, is held back or let go by the shoot at the top, and is built to bend in the wind.
Some trees grow like a rocket, with one main stem and short side branches, like a Christmas tree. Others grow into a big round broccoli shape. It depends on how bossy the top shoot is and how much light each branch gets.
Grown-up words: excurrent form is the grown-up word for ‘grow like a rocket’ decurrent form is the grown-up word for ‘a big round broccoli shape’ apical control is the grown-up word for ‘how bossy the top shoot is’
Each bud is a tiny folded-up shoot with the baby leaves already inside, wrapped in scales like a sleeping bag. In spring it just unpacks. Some trees can then grow extra new leaves on top later in the summer.
Grown-up words: preformed growth is the grown-up word for ‘the baby leaves already inside’ bud scales is the grown-up word for ‘wrapped in scales’ lammas growth is the grown-up word for ‘grow extra new leaves on top later in the summer’
Trees hide spare buds under their bark for years and years. If the tree loses branches, gets hurt or suddenly gets lots of light, these wake up and shoot out.
Grown-up words: dormant buds is the grown-up word for ‘spare buds under their bark’ epicormic shoots is the grown-up word for ‘wake up and shoot out’
Branches and trunks are meant to bend. Swaying spreads out the push of the wind so the tree doesn’t snap. A tree that gets blown about a lot grows thicker and stronger, just like your muscles when you exercise.
Grown-up words: damping is the grown-up word for ‘Swaying spreads out’ wind loading is the grown-up word for ‘the push of the wind’ thigmomorphogenesis is the grown-up word for ‘grows thicker and stronger’
If a trunk or branch leans, the tree grows special wood to hold it up. Broadleaf trees pull it up from the top side like a rope. Conifers push it up from underneath like a prop.
Grown-up words: reaction wood is the grown-up word for ‘special wood to hold it up’ tension wood is the grown-up word for ‘pull it up from the top side like a rope’ compression wood is the grown-up word for ‘push it up from underneath like a prop’
Grown-up words: the special wood is reaction wood. The rope on top of a broadleaf is tension wood; the prop underneath a conifer is compression wood.
Trees drop branches they don’t need any more, like shady ones low down. They seal the branch off first, so when it falls off, the hole is already protected.
Grown-up words: natural branch shedding is the grown-up word for ‘drop branches they don’t need any more’ branch protection zone is the grown-up word for ‘seal the branch off first’
At the bottom of every branch is a little swollen collar. Inside, the branch wood and the trunk wood wrap round each other like fingers laced together. That joint is where a tree is best at fighting off germs.
Grown-up words: branch collar is the grown-up word for ‘a little swollen collar’ branch union is the grown-up word for ‘wrap round each other like fingers laced together’
When tree people cut off a branch, they cut just outside that collar: not flat against the trunk, and not leaving a long stump. That way the tree can seal the wound up properly.
Grown-up words: target pruning is the grown-up word for ‘cut just outside that collar’ flush cut is the grown-up word for ‘flat against the trunk’ stub is the grown-up word for ‘a long stump’
Just right. The collar is left whole, so the tree can wall off the cut and roll new wood over it.
Grown-up words: cutting just outside the collar is target pruning, and it keeps the branch protection zone. A cut flat against the trunk is a flush cut; a long leftover bit is a stub.
If two big stems are joined weakly, a tree surgeon might fit a strong rope or cable between them, like a seatbelt, to stop them splitting apart in a storm.
Grown-up words: crown support system is the grown-up word for ‘a strong rope or cable’ dynamic bracing is the grown-up word for ‘like a seatbelt’
Inside the trunk
Water pipes, food tubes, a thin growing layer and a coat of bark. The trunk holds the tree up and moves everything around it.
Just under the bark is a layer thinner than paper that never stops making new cells. On the inside it makes wood. On the outside it makes food tubes and bark. That’s how a trunk gets fatter every single year.
Grown-up words: vascular cambium is the grown-up word for ‘a layer thinner than paper’ secondary xylem is the grown-up word for ‘it makes wood’ secondary phloem is the grown-up word for ‘food tubes’
Tap a layer to find out what it does.
Grown-up words: the outer bark is the periderm, the food tubes are secondary phloem, the growing layer is the vascular cambium, the pale lines are rays, and the soft middle is the pith.
In spring, a tree makes big, open wood cells fast. By late summer it makes small, thick ones. Where the small ones meet next spring’s big ones you get a line. One ring for every year.
Grown-up words: earlywood is the grown-up word for ‘big, open wood cells’ latewood is the grown-up word for ‘small, thick ones’ growth ring is the grown-up word for ‘One ring for every year’
Most of the cells in wood are actually dead! They die on purpose, leaving behind empty pipes and strong walls. Dead, but still holding the tree up and carrying water.
Grown-up words: programmed cell death is the grown-up word for ‘die on purpose’ vessels and tracheids is the grown-up word for ‘empty pipes’ fibres is the grown-up word for ‘strong walls’
Threaded all through the wood are rows of cells that stay alive. They store food, carry things sideways across the trunk, and help fight germs.
Grown-up words: parenchyma is the grown-up word for ‘rows of cells that stay alive’ rays is the grown-up word for ‘carry things sideways across the trunk’
Water travels from the roots up to the leaves through millions of tiny pipes in the wood. Conifers have thin, safe pipes. Trees like oak have big, fast pipes.
Grown-up words: xylem is the grown-up word for ‘millions of tiny pipes in the wood’ tracheids is the grown-up word for ‘thin, safe pipes’ vessels is the grown-up word for ‘big, fast pipes’
A tree has no heart to pump water. Instead, when water evaporates out of the leaves, it tugs on the water behind it, like sucking on a very long straw. Water holds on to itself so well that the whole thin line gets pulled up, even to the top of trees over 100 metres tall.
Grown-up words: transpiration is the grown-up word for ‘water evaporates out of the leaves’ tension is the grown-up word for ‘tugs on the water behind it’ cohesion is the grown-up word for ‘Water holds on to itself’
If the pull gets too strong in a drought, or water freezes and then thaws, an air bubble can pop into a pipe and block it, like a bubble in a straw. Big pipes block more easily. Block too many and branches die.
Grown-up words: cavitation is the grown-up word for ‘an air bubble can pop into a pipe’ embolism is the grown-up word for ‘block it’ hydraulic failure is the grown-up word for ‘Block too many and branches die’
A normal day: water is pulled up all three pipes.
Grown-up words: an air bubble blocking a pipe is an embolism, and air snapping in is cavitation. Wider pipes are more at risk from both drought and freeze–thaw cavitation.
A tree can’t have pipes that are both super-fast and super-safe. Big pipes move lots of water but break easily. Small pipes are slow but safe. Every kind of tree picks its own mix.
Grown-up words: hydraulic safety–efficiency trade-off is the grown-up word for ‘both super-fast and super-safe’
The outside, younger wood carries water and still has living cells. The middle, older wood stops working as pipes and often fills up with dark chemicals that make it hard for rot to get in.
Grown-up words: sapwood is the grown-up word for ‘The outside, younger wood’ heartwood is the grown-up word for ‘The middle, older wood’ extractives is the grown-up word for ‘dark chemicals’
Water from one root mostly goes straight up one side of the tree, to the branches on that side. So hurting the roots on one side can make the branches on that same side struggle.
Grown-up words: vascular sectorality is the grown-up word for ‘straight up one side of the tree’
Sugar made in the leaves gets delivered all round the tree through living food tubes just under the bark. It goes wherever it’s needed: down to the roots, into fruits, out to new shoots.
Grown-up words: phloem is the grown-up word for ‘living food tubes just under the bark’ source and sink is the grown-up word for ‘wherever it’s needed’
If you cut a ring all the way round the bark, the food tubes are cut. Water can still go up, but sugar can’t get down, and the roots slowly starve. That’s why bark damage matters so much.
Grown-up words: girdling is the grown-up word for ‘cut a ring all the way round the bark’ root starvation is the grown-up word for ‘the roots slowly starve’
Bark is the tree’s raincoat and armour. The outside is dead cork, which keeps water in and germs out. As the trunk gets fatter, the old bark cracks and a new layer grows underneath. Little breathing holes let air through.
Grown-up words: cork (phellem) is the grown-up word for ‘dead cork’ rhytidome is the grown-up word for ‘the old bark cracks’ cork cambium (phellogen) is the grown-up word for ‘a new layer grows underneath’ lenticels is the grown-up word for ‘Little breathing holes’
Trees use sugar a bit like you use food: every living cell ‘burns’ it slowly to get energy for growing and fixing things. So every living part of a tree breathes out a little carbon dioxide, day and night.
Grown-up words: respiration is the grown-up word for ‘‘burns’ it slowly to get energy’ growth and maintenance respiration is the grown-up word for ‘growing and fixing things’
Trees save spare sugar as starch in their wood and roots, like pocket money. In spring they spend it opening buds before the new leaves can make any food. If a tree keeps losing its leaves, the piggy bank runs empty.
Grown-up words: carbohydrate reserves is the grown-up word for ‘save spare sugar as starch’ remobilisation is the grown-up word for ‘spend it opening buds’
Summer: the leaves are the sugar makers. Sugar flows down to the roots and out to fruit and new wood, and some is saved up.
Grown-up words: the food tubes are phloem. Where sugar comes from is the source, where it goes is the sink, and the savings are carbohydrate reserves.
Trees usually die slowly, from a mix of problems: pipes blocked by bubbles, the piggy bank running empty, then bugs and fungi moving in on a weak tree.
Grown-up words: hydraulic failure is the grown-up word for ‘pipes blocked by bubbles’ carbon starvation is the grown-up word for ‘the piggy bank running empty’ decline spiral is the grown-up word for ‘bugs and fungi moving in on a weak tree’
Just after the new leaves open in spring, a tree has spent most of its savings and hasn’t earned them back yet. Cutting lots off then hurts it most.
Grown-up words: leaf flush is the grown-up word for ‘Just after the new leaves open in spring’ carbohydrate reserves is the grown-up word for ‘savings’
Falling to the ground
They make flowers, the flowers make seeds, and the seeds travel away to grow. People help by growing and planting young trees carefully.
Pollen from one flower has to get to another flower. Some trees use bees to carry it. Others, like birch and oak, just throw clouds of pollen into the wind and hope.
Grown-up words: pollination is the grown-up word for ‘Pollen from one flower has to get to another flower’ insect pollination is the grown-up word for ‘use bees to carry it’ wind pollination is the grown-up word for ‘throw clouds of pollen into the wind’
Many trees have boy flowers and girl flowers on the same tree. Some, like holly, have boy trees and girl trees, and only the girl trees get berries.
Grown-up words: monoecious is the grown-up word for ‘boy flowers and girl flowers on the same tree’ dioecious is the grown-up word for ‘boy trees and girl trees’
A seed is a tiny baby tree, folded up with a packed lunch, inside a tough coat. It can wait a long time until the right moment to grow.
Grown-up words: embryo is the grown-up word for ‘a tiny baby tree’ endosperm or cotyledons is the grown-up word for ‘a packed lunch’ seed coat is the grown-up word for ‘a tough coat’
Trees make huge numbers of seeds because almost all of them get eaten or land somewhere hopeless. Some years a tree makes an enormous amount all at once, so the animals can’t possibly eat them all.
Grown-up words: seed predation is the grown-up word for ‘get eaten’ masting is the grown-up word for ‘makes an enormous amount all at once’
Fruits help seeds travel. Sycamore helicopters spin away on the wind, birds eat berries and poo the seeds out somewhere new, and squirrels carry nuts off and bury them.
Grown-up words: samaras is the grown-up word for ‘Sycamore helicopters’ animal dispersal is the grown-up word for ‘birds eat berries and poo the seeds out’ scatter-hoarding is the grown-up word for ‘squirrels carry nuts off and bury them’
Lots of seeds won’t grow straight away. They need to feel a cold winter, or have their coat cracked, first. That way they wake up in spring and not just before winter.
Grown-up words: seed dormancy is the grown-up word for ‘won’t grow straight away’ cold stratification is the grown-up word for ‘feel a cold winter’ scarification is the grown-up word for ‘have their coat cracked’
When a seed wakes up, it drinks water and swells. The root pops out first to grab the ground. Then the shoot heads up to find the light.
Grown-up words: germination is the grown-up word for ‘When a seed wakes up’ imbibition is the grown-up word for ‘drinks water and swells’ radicle is the grown-up word for ‘The root pops out first’
Step 1: a grown-up tree makes flowers.
Grown-up words: pollen travelling is pollination, the seed forming is fertilisation, the seed travelling is seed dispersal, the seed sleeping is dormancy, the seed waking is germination, and the sapling taking hold is establishment.
Some trees make copies of themselves by sending up new shoots from their roots, like a whole family of twins. Gardeners copy trees too, using cuttings and by joining one tree onto another’s roots.
Grown-up words: root suckers is the grown-up word for ‘sending up new shoots from their roots’ clones is the grown-up word for ‘a whole family of twins’ grafting is the grown-up word for ‘joining one tree onto another’s roots’
Planting a tree is like picking a pet: you need the right one for the space. A huge tree in a tiny garden means trouble later on.
Grown-up words: species selection is the grown-up word for ‘the right one for the space’ right tree, right place is the grown-up word for ‘A huge tree in a tiny garden means trouble’
Lots of young trees start life in a tree nursery. A good one has a strong stem and healthy roots that don’t go round and round the pot.
Grown-up words: nursery stock is the grown-up word for ‘young trees start life in a tree nursery’ circling roots is the grown-up word for ‘roots that don’t go round and round the pot’
Plant a tree at the same depth it was growing, with the flare at the bottom of the trunk showing. Water it, put a blanket of wood chip round it, and don’t tie it up too tight. The first few years are the hardest.
Grown-up words: planting depth is the grown-up word for ‘the same depth it was growing’ root flare is the grown-up word for ‘the flare at the bottom of the trunk’ mulch is the grown-up word for ‘a blanket of wood chip’ staking is the grown-up word for ‘tie it up’ establishment is the grown-up word for ‘The first few years’
Under the ground
They hold the tree up, drink water, collect minerals from the soil, and team up with fungi to reach even further.
Most roots aren’t very deep. They stay in the top part of the soil, where there’s air and water. But they spread a long way, often well past the edge of the branches.
Grown-up words: rooting depth is the grown-up word for ‘Most roots aren’t very deep’ root spread is the grown-up word for ‘they spread a long way’ root protection area is the grown-up word for ‘well past the edge of the branches’
Lots of people imagine roots as a mirror of the branches, going deep down.
Grown-up words: how deep roots go is rooting depth, how far they reach is root spread, and the area kept safe on building sites is the root protection area.
Every root tip wears a little cap that pushes through the soil. Just behind it are tiny fuzzy hairs that soak up water.
Grown-up words: root cap is the grown-up word for ‘a little cap’ root hairs is the grown-up word for ‘tiny fuzzy hairs’
Thick roots near the trunk work like tent ropes and pegs, stopping the tree from blowing over. They taper quickly as they head away from the trunk.
Grown-up words: structural roots is the grown-up word for ‘Thick roots near the trunk’ root plate is the grown-up word for ‘tent ropes and pegs’ zone of rapid taper is the grown-up word for ‘taper quickly’
The roots that actually drink are thinner than spaghetti and only live a short while. The tree keeps growing new ones all the time.
Grown-up words: fine roots is the grown-up word for ‘thinner than spaghetti’ root turnover is the grown-up word for ‘only live a short while’
Roots don’t grow all year. Lots of trees grow new roots in spring, often before the leaves come out, and again in autumn.
Grown-up words: root phenology is the grown-up word for ‘in spring, often before the leaves come out, and again in autumn’
Soil needs tiny gaps for air and water. Cars, diggers and lots of feet squash those gaps shut, and then roots can’t breathe or push through.
Grown-up words: soil pores is the grown-up word for ‘tiny gaps for air and water’ soil compaction is the grown-up word for ‘squash those gaps shut’ poor aeration is the grown-up word for ‘can’t breathe’ penetration resistance is the grown-up word for ‘push through’
Loose soil: grains with gaps between them. Some gaps hold air, some hold water, and the root can wriggle through.
Grown-up words: squashed soil is compacted soil. The gaps are pores, and how hard it is for a root to push through is penetration resistance.
A layer of wood chip on the soil keeps it soft, damp and full of worms. It’s like a warm blanket for roots.
Grown-up words: mulching is the grown-up word for ‘A layer of wood chip on the soil’ soil biology is the grown-up word for ‘full of worms’
In towns, trees often get a tiny hole of soil in the pavement. A big tree needs a big space for roots, like a fish needs a big enough tank.
Grown-up words: tree pit is the grown-up word for ‘a tiny hole of soil in the pavement’ rooting volume is the grown-up word for ‘a big space for roots’
Soil can feel damp but still be hard to drink from, because it holds on tight to the last bits of water. What matters is how hard the tree has to suck.
Grown-up words: matric potential is the grown-up word for ‘holds on tight to the last bits of water’ water potential is the grown-up word for ‘how hard the tree has to suck’
Water always moves from ‘wetter’ places to ‘drier’ places: from soil, into root, up to leaf, out into the air. Dissolved stuff inside cells helps pull water in too.
Grown-up words: water potential gradient is the grown-up word for ‘from ‘wetter’ places to ‘drier’ places’ soil–plant–atmosphere continuum is the grown-up word for ‘from soil, into root, up to leaf, out into the air’ osmosis is the grown-up word for ‘Dissolved stuff inside cells helps pull water in’
Deep inside each root is a special ring of cells that works like a security guard, checking everything before it’s allowed in.
Grown-up words: endodermis is the grown-up word for ‘a special ring of cells’ Casparian strip is the grown-up word for ‘checking everything before it’s allowed in’
Roots use energy to pump in the minerals they need, even when there isn’t much in the soil.
Grown-up words: active transport is the grown-up word for ‘use energy to pump in’ mineral nutrients is the grown-up word for ‘the minerals they need’
Trees need minerals from the soil, a bit like you need vitamins. Nitrogen for leaves, phosphorus for energy, potassium for opening and shutting leaf mouths, and tiny bits of others like iron. Too little and leaves go yellow or spotty.
Grown-up words: macronutrients is the grown-up word for ‘Nitrogen for leaves, phosphorus for energy, potassium’ micronutrients is the grown-up word for ‘tiny bits of others like iron’ chlorosis is the grown-up word for ‘leaves go yellow’
Trees take in nitrogen and build it into the pieces that proteins are made from. It takes a lot of energy to do.
Grown-up words: nitrogen assimilation is the grown-up word for ‘take in nitrogen and build it’ amino acids is the grown-up word for ‘the pieces that proteins are made from’
Most tree roots are wrapped in helpful fungi. The fungus threads reach much further than roots can, bringing back water and minerals. The tree pays them in sugar. It’s a swap!
Grown-up words: mycorrhizas is the grown-up word for ‘helpful fungi’ hyphae is the grown-up word for ‘The fungus threads’ symbiosis is the grown-up word for ‘It’s a swap!’
On its own, a root can only drink from the soil just around it.
Grown-up words: the root-and-fungus team is a mycorrhiza, the threads are hyphae, and the swap is a symbiosis.
Alder trees have little lumps on their roots full of tiny living things that can grab nitrogen straight out of the air and share it with the tree.
Grown-up words: root nodules is the grown-up word for ‘little lumps on their roots’ Frankia bacteria is the grown-up word for ‘tiny living things’ nitrogen fixation is the grown-up word for ‘grab nitrogen straight out of the air’
Whether soil is sour or chalky changes which minerals roots can get. Some trees, like pin oak and sweetgum, go yellow in chalky soil because they can’t get enough iron.
Grown-up words: soil pH is the grown-up word for ‘sour or chalky’ lime-induced chlorosis is the grown-up word for ‘go yellow in chalky soil’
When leaves fall and rot, worms, fungi and bugs break them down and put the goodness back in the soil for the tree to use all over again.
Grown-up words: leaf litter is the grown-up word for ‘leaves fall and rot’ decomposers is the grown-up word for ‘worms, fungi and bugs’ nutrient cycling is the grown-up word for ‘put the goodness back in the soil’
At night, some roots move water from wet, deep soil up into dry soil near the top, a bit like watering their own garden.
Grown-up words: hydraulic redistribution is the grown-up word for ‘move water from wet, deep soil up into dry soil’
Most trees don’t need extra plant food. Too much can even do harm. It’s better to test the soil first, and fix squashed or poor soil, than to pour food on.
Grown-up words: fertiliser is the grown-up word for ‘extra plant food’ soil analysis is the grown-up word for ‘test the soil first’
When things go wrong
They either dodge the problem or tough it out, standing in one spot the whole time.
Trees handle hard times in two ways. They dodge the problem, like dropping leaves before a drought gets bad. Or they tough it out, like having extra-strong pipes. Some trees can get tougher if they’re given practice.
Grown-up words: stress avoidance is the grown-up word for ‘dodge the problem’ stress tolerance is the grown-up word for ‘tough it out’ acclimation is the grown-up word for ‘get tougher if they’re given practice’
In autumn, trees toughen up a little bit each week so they can survive freezing. A tree caught by an early frost before it’s ready can get hurt.
Grown-up words: cold acclimation is the grown-up word for ‘toughen up a little bit each week’ cold hardiness is the grown-up word for ‘survive freezing’
When it freezes, trees let ice form in the gaps between their cells, not inside them. Ice inside a cell would burst it, like a bottle of water left in the freezer.
Grown-up words: extracellular freezing is the grown-up word for ‘ice form in the gaps between their cells’ intracellular freezing is the grown-up word for ‘Ice inside a cell’
A warm day: each cell is full of water.
Grown-up words: ice in the gaps is extracellular freezing, water leaving the cells is freeze-induced dehydration, and ice inside a cell is intracellular freezing.
A frost after the buds have opened can kill the soft new leaves. That’s one reason some trees wait until later in spring to open.
Grown-up words: late spring frost is the grown-up word for ‘A frost after the buds have opened’ late budburst is the grown-up word for ‘wait until later in spring to open’
On hot days, leaves cool themselves by letting water out, a bit like you sweat. If they’re short of water too, they can’t keep cool and they cook.
Grown-up words: transpirational cooling is the grown-up word for ‘cool themselves by letting water out’ heat stress is the grown-up word for ‘they cook’
When water runs out, trees close their leaf mouths, stop growing and may drop leaves early. In a long drought, their water pipes fill up with bubbles.
Grown-up words: stomatal closure is the grown-up word for ‘close their leaf mouths’ drought-induced leaf shedding is the grown-up word for ‘drop leaves early’ embolism is the grown-up word for ‘water pipes fill up with bubbles’
Roots need air to breathe. In waterlogged soil there isn’t any, so roots start to suffocate. Some trees, like willows and swamp cypress, have tricks: air tubes inside their roots, and even roots that poke up out of the water.
Grown-up words: hypoxia is the grown-up word for ‘roots start to suffocate’ aerenchyma is the grown-up word for ‘air tubes inside their roots’ pneumatophores is the grown-up word for ‘roots that poke up out of the water’
Salt put on icy roads soaks into the soil. It makes it hard for roots to drink and poisons the leaves, which go brown round the edges.
Grown-up words: osmotic stress is the grown-up word for ‘hard for roots to drink’ ion toxicity is the grown-up word for ‘poisons the leaves’ marginal leaf scorch is the grown-up word for ‘brown round the edges’
A tree can often cope with one problem. Drought plus squashed soil plus a hungry bug is much harder. Troubles add up.
Grown-up words: predisposition is the grown-up word for ‘Drought plus squashed soil’ stress interactions is the grown-up word for ‘Troubles add up’
Back up in the daylight
They cool our streets, soak up rain, clean the air and store carbon, and they need people who understand them to look after them.
Trees give shade, cool down hot streets, soak up heavy rain, clean the air, lock carbon away in their wood, and make people feel happier.
Grown-up words: canopy shading is the grown-up word for ‘give shade’ urban cooling is the grown-up word for ‘cool down hot streets’ rainfall interception is the grown-up word for ‘soak up heavy rain’ carbon sequestration is the grown-up word for ‘lock carbon away in their wood’
No tree: the sun heats the pavement, and the pavement heats the air. The thermometer climbs.
Grown-up words: a town being hotter than the countryside is the urban heat island, and leaves ‘sweating’ is evapotranspiration.
Tree surgeons use all of this science to keep trees healthy and safe. Knowing how a tree works tells you where to cut, how to plant, and why roots and soil need protecting.
Grown-up words: arborists is the grown-up word for ‘Tree surgeons’ arboriculture is the grown-up word for ‘all of this science’
That’s 121 questions about how trees work.
The topics follow two books I absolutely love to bury my head in when I’ve got time for learning: Applied Tree Biology by Andrew Hirons and Peter Thomas (Wiley, 2018), and Plant Physiology by Lincoln Taiz and Eduardo Zeiger. If this page has got you hooked, they’re the grown-up version, and they’re brilliant. No connection, I just rate them.