What speeds up transpiration.

Step 1 of 6

Dry air

Inside the leaf, in the spaces between the spongy cells, the air is saturated with water vapour: it holds as much as it can. The vapour leaves through the stomata by diffusion, from where there is plenty of vapour to where there is less, and the bigger the difference, the faster it goes. Washing dries faster on a dry day for the same reason.

The chart beside the Scene shows the same thing in the language of water potential: water always flows from a higher potential to a lower one, from the moist soil, through the roots and the trunk, to the leaf and from there to the air. Predict what happens when the air dries out.

Step 2 of 6

Muggy air

Now the opposite: very moist air, like a summer day at the seaside. When the leaf evaporates less, it also cools less, because evaporation takes heat. Watch the leaf’s temperature on the chart as the humidity changes.

Step 3 of 6

Half the flow

Sunlight and Wind are fixed now, and only Humidity is free. If transpiration depends on the difference between the saturated air in the leaf and the air outside, how humid must the air be to halve it? Guess first, then search with the chart.

Step 4 of 6

Wind

On a leaf in still air sits a thin layer of moist air that does not move, and in the leaf cut across you see it as Still air. The vapour has to cross it too. Here the air is still; predict what a strong wind will do.

Step 5 of 6

Saving water

The stomata are a trade-off. Open, they let carbon dioxide in for photosynthesis and water out. When the leaf loses water faster than the xylem brings it, its water potential falls, and the guard cells start to close the stomata, even while the sun is still strong. All three controls are free: find when that happens. On the chart the dashed mark shows how far the light alone would open the stomata.

Step 6 of 6

The whole tree

The chart gives two numbers: the rate of loss from each square metre of leaf, in millimoles of water a second, and the water the whole tree lifts, in litres an hour. You can work out the second from the first, if you know the leaf area and the mass of a mole of water, and practise converting units on the way.