The water, containing salts, which is gathered by the roots is brought up to the leaves.

Here it combines with the carbonic-acid gas taken from the air.

Under the action of chlorophyll and sunlight these substances are split up, the carbon, oxygen and hydrogen being combined into plant food.

It is either used immediately or stored away for future emergency.
Trees breathe somewhat like human beings.

They take in oxygen and give off carbonic-acid gas.