Plant Transport

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Growing tall gives plants huge advantages like access to more sunlight.

But it also brings huge challenges.

How this towering tree gets water from the ground up to its leaves, seems an extraordinary feat of engineering.

Plants have a simple system of tubes that carry water and food around the plant.

The xylem and phloem.

Xylem
Phloem

Towards the outside of the stem, are the phloem tubes.

Smaller than the width of a hair, they're packed closely together, and run the whole length of the plant.

Each tube is made of living cells, joined end to end, forming a continuous tube.

Their thin walls are perforated, like a sieve, to allow sugars made in the leaves to flow to all the other parts of the plant.

Phloem:
Living cells
Transport food from leaves

Further inside the stem is the xylem – dead cells that form a hollow tube.

Water and minerals are drawn up inside these tubes, from the roots to the leaves.

Xylem:
Dead cells
Transport water from roots to leaves

But how does this happen?

Deep underground, root hair cells project into the soil, seeking moisture.

Because the inside of the cell contains less water than the soil solution, water moves into the root hair cells by osmosis.

And towards the xylem in the root.

Osmosis:
Movement from high to low concentration

But once in the xylem, another force comes into play.

It's generated right at the top of the plant.

Transpiration: the evaporation of water from leaves.

Transpiration:
90% of water from roots lost through leaves

As water leaves the leaf cells, more water is drawn out of the xylem to replace it and because it forms a continuous tube, the hollow xylem acts like a thin drinking straw, sucking water up from the roots.

So the secret to how water can move upwards, is that it's pulled, not pushed.

The powerful suction force generated by transpiration can draw water to the tops of the tallest trees.