Every living cell in a body needs oxygen, nutrients and other materials in order to stay healthy and alive, and every cell also produces substances that are of no use to it. Carrying the materials a cell needs to that cell, and carrying the unwanted materials from the cell to the organs that get rid of them, is called transportation. Plants and animals do this work in completely different ways.
The textbook opens with two ordinary questions that are worth sitting with. Once food has been digested, how does it actually reach every cell? And if a tablet is swallowed into the stomach, how does it end up working on a cut on your finger? Both questions have the same answer, and the answer is transportation.
A tall tree has no heart. So how does water reach the top?
Here is the question the textbook does not ask, and it is the best one in the topic. Water enters a tree at the roots, in the ground. The leaves are twenty or thirty metres above, and in some trees far higher than that. A plant has no heart, no pump and no muscle of any kind. Nothing in it is pushing. So what carries water all that way upwards, against gravity, every single day?
The answer is the opposite of what most people guess. The roots are not pushing water up. The leaves are pulling it up, and they are doing it simply by losing water into the air.
| Step | What happens |
|---|---|
| 1 | Water evaporates out of a leaf through the tiny openings called stomata. This is transpiration, and on a hot day a large tree loses a great deal of water this way. |
| 2 | That leaves a gap at the top of the water column inside the leaf. |
| 3 | Water molecules stick to one another. So as one leaves, it drags the next one up behind it, and that one drags the next, all the way down the tube. |
| 4 | The whole column of water in the xylem is pulled upwards like a rope being hauled from above, and fresh water is drawn in at the root hairs to replace it. |
So a tree is not a pump. It is closer to a wick with a fan at the top. The water is not being pushed from below, it is being drawn out at the top, and everything below follows because water holds on to itself. This also explains something you have already seen. A plant left in strong sun with dry soil wilts, because water is still being pulled out of the leaves and there is nothing left below to follow it up.
Two sets of pipes, and the difference is direction
A plant has two separate transport tissues and it is worth understanding why one would not have been enough. Xylem carries water and dissolved minerals, which the roots have absorbed from the soil, up to the stem, the branches and the leaves. Phloem carries the food made in the leaves and other green parts out to every part of the plant.
| Xylem | Phloem | |
|---|---|---|
| Carries | Water and dissolved minerals, and dissolved gases | Food made by the green parts of the plant |
| From where to where | Roots upwards only | Leaves outwards, upwards to buds and downwards to roots |
| Direction | One way | Both ways |
| What the tube is made of | Dead, empty cells joined end to end, so nothing blocks the flow | Living cells, because moving food about is work |
The row that matters is direction. Water only ever has one journey to make, from the soil to the leaves, so a one way pipe is enough. Food is different. It is made in the leaves, and it may be needed by a bud growing above the leaf or by a root buried below it, so it has to be able to travel both ways. That is the reason a plant needs two systems rather than one.
Here is a test of whether the last table is understood. If a ring of bark is stripped all the way round a young tree, by a goat or by a knife, the tree does not die that week. The leaves stay green for a while and may even grow. Then, over months, the whole tree fails. Why the delay? Because the phloem is in the bark and the xylem is deeper in the wood. Water can still be pulled up past the wound, so the leaves go on working. But no food can travel down past the ring, so the roots slowly starve, and when the roots stop working the water stops too. This is exactly why grazing animals in a young plantation cause damage that nobody notices for a season, and it connects straight back to the grazing problem in Unit 4.
How water gets into the plant in the first place
Water is absorbed from the soil by the roots, and in particular by the root hairs, which are the thin outgrowths near the tip of every young root. You met these in Unit 3, and the reason for them was the same idea that keeps coming back in biology. A root hair is a surface trick. Growing thousands of very thin hairs gives a root an enormous surface touching the soil water, without the root itself having to be enormous. Every drop of water in a tall tree came in through one of those hairs.
Gases move about inside a plant as well. Oxygen and carbon dioxide enter through fine openings such as the stomata, dissolve in the water inside the plant, and are then carried along in the xylem with everything else. That is worth noticing, because it means a plant has no separate breathing pipes at all. The same water that feeds the plant is also the water its gases travel in.
Do not write that roots push water up the plant. Nothing in a plant pushes. The correct wording is that water is pulled up as it evaporates from the leaves, and that the column holds together because water molecules stick to one another. If you can say that sentence, you can also explain why a plant wilts on a hot day, why cutting a stem under water keeps a flower fresh longer, and why a tree loses more water on a windy afternoon than on a still one.
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