How a tall plant drinks without a heart

How to use this simulation

1. Look at the plant on the left. Blue dots climb the xylem and orange dots descend the phloem. Small puffs of vapour leave the stomata on the underside of the leaf. The stoma close-up on the right shows the same thing at a larger scale.
2. Drag the light slider to zero. The guard cells close the pore, the transpiration bar collapses, the xylem dots slow down and the sugar flow in the phloem falls to nothing.
3. Drag the light slider back up and then lower the humidity. The air outside is drier, so more water leaves each stoma and the rate reading rises.
4. Raise the wind speed. The arrows on the left multiply and the vapour is carried away from the leaf, so the rate climbs even though the stomata are no wider.
5. Watch the Main driver reading as you move each slider. It names whichever of the three factors is currently doing most.
6. Press the reset button at the top to return to the default day and compare your numbers with the solved problems below.

The tall tree problem

A mango tree can be twenty metres tall. Every drop of water in its top leaves arrived from a root hair in the soil, climbing all that way with no heart and no pump. A person’s heart pushes blood only a metre or two, and even that takes a muscle the size of a fist.

So how does the tree do it? The surprising answer is that the plant does not push the water at all. It pulls. The pull starts in the leaf, where water evaporates through tiny pores, and it is transmitted down a continuous thread of water all the way to the roots.

At the same time something quite different is going on in a neighbouring tube. The leaf has just made sugar by photosynthesis and has more than it needs. Sugar flows out of the leaf and down to the roots, fruits and growing tips that cannot make their own. This second flow is not pulled by evaporation and does not work the same way.

This page lets you play with both flows. You will change the light, the humidity and the wind, watch the stomata open and close, and calculate the transpiration rate with the same arithmetic as the readings panel.

Two tubes, two jobs

Every flowering plant has a plumbing system called vascular tissue. It has two parts that run side by side in the root, the stem and the veins of the leaf, so one cut across a stem shows them as bundles.

Words you need first

Plant transport is full of look-alike terms. Learn them in pairs.

Water moves up the xylem: cohesion and tension

Follow a water molecule from the soil to the sky in four steps. Each one is visible in the simulation.

What sets the transpiration rate

The simulation uses a deliberately simple model so you can see how three factors combine. Rate = 150 × stomatal opening × humidity deficit × wind factor, in grams of water per square metre of leaf per hour. All three of them can be changed with the sliders.

Light controls the stomata. In light the guard cells fill with water and bow apart, widening the pore; in the dark they go slack and the pore shuts. The model lets the stomata keep 10 % of their opening even in total darkness and rise to 100 % in full light, so on the default 70 % light they are 73 % open.

Humidity sets the gradient. The air inside the leaf is saturated. The drier the air outside, the steeper the drop in water vapour from inside to outside and the faster the diffusion. At 60 % humidity the deficit is 40 %.

Wind removes the damp layer that clings to the leaf. Without wind that layer thickens, the gradient flattens and transpiration slows. The model multiplies the rate by 1 + wind ÷ 20, which is 1.25 at 5 km/h.

• Temperature also raises transpiration, because warmer air holds more vapour and evaporation speeds up. The model leaves it out so the three sliders stay readable.
• In real plants a water shortage closes the stomata whatever the light does. This protects the plant but also stops CO₂ from entering, so photosynthesis slows.

Sugar moves down the phloem: source to sink

A mature leaf is a source: it makes more sugar than it uses. A root, a fruit or a growing bud is a sink: it uses or stores sugar and cannot make enough of it. Phloem carries sugar from the sources to the sinks, and which direction that is depends on where the nearest source and sink are. In a young seedling the cotyledons feed the growing shoot; in a mature tree the leaves feed the roots and fruits.

The accepted explanation is the pressure-flow hypothesis. Companion cells use ATP to pump sucrose into the sieve tubes at the source. Water follows by osmosis and raises the pressure there. At the sink the sugar is taken out for use or storage, water leaves, and the pressure falls. The sap flows down the pressure gradient, from high to low, like water in a pipe between a tank and a tap.

The simulation makes sugar flow proportional to photosynthesis, which needs both light and open stomata (for CO₂). On the default day that is 51 % of the maximum.

Try it in the simulation

Spend ten minutes on these experiments. Each one is a question an examiner could ask.

• Set light to 0 and read the stomata and the rate. Is transpiration exactly zero? Why not?
• Set light to 100 and humidity to the minimum, wind to the maximum. What is the highest rate the model can produce, and what is the xylem speed?
• Keep light and wind fixed and move humidity from 100 % to 10 %. How does the rate change?
• Set wind to 0, then to 20 km/h. By what factor does the rate change?
• Move only the light slider and watch the sugar flow reading. Why does it fall faster than the stomatal opening does?
• Hide the labels and name the xylem, phloem, stoma, guard cell and sink from memory, then show them to check.

Solved problems

Each solution states the formula first, then substitutes the numbers. Every figure is computed by the same code that drives the simulation.

Common mistakes

These are the slips that cost marks every year.

• Saying the xylem is pushed up by root pressure alone. Root pressure helps at night in small plants; the main force is transpiration pull.
• Saying phloem flows only downward. It flows from source to sink, which may be up (to a fruit) or down (to a root).
• Writing that xylem is living. Mature xylem vessels are dead, hollow tubes.
• Confusing translocation with transpiration. One moves sugar in the phloem, the other loses water vapour.
• Forgetting that stomata open in light and close in dark (guard cells swell with water and bow apart in light).
• Saying humidity makes the stomata close. It changes the gradient, not the opening.

Where you see it in real life

Celery stalks left in coloured water turn colour along their veins within hours: the dye shows the xylem. A ring of bark cut around a branch (girdling) kills the part beyond it because it cuts the phloem, even though the xylem is untouched.

Wilting on a hot windy afternoon is transpiration running ahead of supply. The leaf loses water faster than the roots can send it, the cells lose pressure and the leaf droops. Watering the base brings it back within an hour.

Sugar-cane and date farmers tap the phloem sap, and aphids pierce a single sieve tube with their mouthparts to feed. Greenhouses are kept humid partly to keep transpiration, and thus the water bill, in check.

Exam corner

Typical questions, and what a full-mark answer contains.

Revision summary

Water enters root hairs by osmosis, rises through dead xylem vessels pulled by transpiration and held together by cohesion, and leaves as vapour through stomata. The rate rises with light (stomata open), falls with humidity (smaller gradient) and rises with wind (damp air removed). The default day gives 54.8 g/m²/h, 109.5 mL/h for the whole plant. Sugar made in source leaves travels in living phloem sieve tubes to sinks by pressure flow, with ATP used for loading.

Frequently asked questions