Leaves are food factories

Animals get their food from plants, directly or indirectly. But plants do not eat. So where does their food come from?

Plants store food as starch, a type of carbohydrate. This starch is made in the leaves, which are usually broad and flat — a shape that catches plenty of sunlight.

Leaves are mostly green because of a green pigment (colouring substance) called chlorophyll. Chlorophyll has two jobs: it gives the leaf its green colour, and it captures sunlight, the energy the leaf needs to make food.

Activity 10.2 tests a leaf for starch:

• Boil the leaf in water for five minutes to soften it.
• Put it in a test tube of alcohol, and stand that test tube in a beaker of boiling water until the leaf loses its green colour.
• Lay the leaf on a plate and add a few drops of dilute iodine solution.

If the leaf turns blue-black, starch is present — the same iodine test you used on rice in the last chapter.

Why remove the green colour first? Because a dark green leaf would hide the blue-black colour; on a pale leaf the change is easy to see.

Safety: alcohol must never be heated directly over a flame. It catches fire very easily and can cause fires and burns. That is why the tube of alcohol is warmed in a beaker of hot water instead.

Because starch is made in them, leaves are called the 'food factories' of plants.