Water through a tree.

Step 1 of 5

A tree that drinks through its leaves

A tree moves nothing, yet it lifts water from the soil ten metres and more, with no pump and no heart. The blue specks climbing from the roots to the leaves in the Scene are that water, and the chart beside the Scene shows how much of it the tree lifts each hour.

Almost none of this water stays in the tree: it leaves the leaves for the air as water vapour. That loss is called transpiration. In this Lesson we follow the water from the root to the leaf and ask what lifts it, and in the next we test what speeds the flow up and what slows it down. Here Sunlight, Humidity and Wind are fixed on a mild, sunny day.

Step 2 of 5

Out through the leaves

A leaf is not a wet sponge. On the outside it is wrapped in a cuticle, a thin layer of wax that lets almost no water through, and inside it are green cells and air spaces saturated with vapour. Answer the question, then move Follow the water down to Leaf to see the leaf cut across: the palisade cells under the upper skin, the loose spongy cells, the vein, and in the lower skin pairs of guard cells round stomata.

Step 3 of 5

Night

The cells inside the leaf need carbon dioxide for photosynthesis, and it comes in through the same stomata the water leaves by. Every open stoma is a bargain: carbon dioxide in, water out. Here you are looking at the leaf cut across, in the daytime sun. Predict what happens when the sun sets.

Step 4 of 5

What lifts the water

Now it is night, and Follow the water down shows the Trunk: the xylem vessels, dead, hollow cells joined end to end into long pipes, each holding a column of water molecules. Look on the chart at how much water flows up the trunk now, and think about what has changed since the day, and what has not.

Step 5 of 5

How fast

The chart also gives the speed of the sap in the trunk, in metres an hour. It rises and falls with transpiration: when the leaves lose more water, the pull is stronger and the water climbs faster. In the Scene the blue specks climb thousands of times faster than in life, so that you can see them move.