Light
Have you made some cakes or biscuits? If you have, you may remember mixing the flour, sugar and water together then putting the mixture in an oven. The things you used to make your cakes or biscuits are called ingredients. When the ingredients were mixed together, you used energy, in the form of heat from the oven, to change the ingredients into something to eat. If you look at any plant leaf in the daylight it is making food using the same processes as you did when you made your cakes or biscuits. It is mixing ingredients and it is using energy to change them into a food.

A plant uses very simple ingredients. They are air and water and nourishment from the soil. It uses energy in the form of sunlight. If you look at any plant in the daytime you might think that it is not doing anything but it is slowly collecting its ingredients and making food. Down in the soil, below the plant stem, the roots are busy sucking up water and nourishment called minerals. These ingredients are moved along tiny pipes inside the root and into tiny pipes in the stem. They move up the stem and out onto the leaf through the leaf veins.

While the water and minerals are moving through the plant, air is entering the leaves. On the underside of the leaves are tiny holes. They are so small that you need a microscope to see them. The holes look like little mouths with a top and bottom lip. At night the mouths close by bringing the lips together, just as you do when you close your mouth. In the day the mouths open and the lips move apart just as yours do when you say "Ah!" to a doctor. When the mouths are open air can move inside the leaf.

When the water and air meet inside a leaf nothing would happen to them if the leaf did not have a green substance in it. This substance traps some of the sunlight shining on the leaf. Sunlight is a form of energy, just like the heat in an oven. In the leaf the energy from sunlight changes the air and water into a food. This food is a kind of sugar. More energy from sunlight is used to change the sugar into other kinds of foods such as starch and oil. Some of the minerals from the soil are used to make more changes and produce the foods which can be used to build new parts of the plant.

When the leaf has made its foods they are moved to other parts of the plant where they are needed. The foods travel in a second set of pipes, which run close to the pipes carrying water and minerals from the roots.

If you watch a chef prepare a meal you may see him or her use many different ingredients and perhaps chop up some vegetables and throw some pieces away. A plant has a similar problem. When it is making its food and changing the air and water it also makes something that it does not need so it throws it away. This unwanted substance is allowed to pass out through the holes in the leaf and drift away into the air. It is a substance that we cannot live without. It is called oxygen. Not all the oxygen that a plant makes leaves the plant. Some of it is used by the plant itself. Plants make much more oxygen than they need so there is always some left over to keep us and other animals alive.

Light, then, is vitally important to life on the Earth. It provides the energy for plants to make food for themselves and for animals. During this food-making process, some of the energy is also used to make oxygen which living things need for survival.

What happens to a plant if it cannot get enough light?
Its leaves go brown and it dies. The plant has no other way of getting the energy it needs.

Why can mushrooms grow in the dark?
They do not make their own food using light energy. They grow very thin white hairs into the soil and feed on the dead plant and animal material that is forming the humus.

How can a seed sprout and grow a seedling when it is underground?
A seed contains a tiny plant and a store of food. In the food store is energy. The tiny plant uses the food and energy to grow a root down into the soil and a shoot up into the light. When the shoot reaches the light it sprouts leaves which turn green and begin to make food for the plant. If the seed is planted too deeply, all the food and energy in the seed may be used by the seedling before the shoot reaches the light. If this happens the seedling dies. That is why it is important that seeds should be planted at the depth stated on the seed packet.

Can a shoot grow after the light?
Yes, it can. If a seedling grows in a shady place it can bend towards the light. This makes more light shine on the leaves and the seedling can make more food. The seedling then uses this food to grow towards the light.

How does the seedling bend its stem?
At the tip of the shoot are substances which make the shoot grow. Oddly enough, the plant produces more of these substances in the dark. If the light shines on one side of the seedling, more growth substances are made on the dark side than on the light side. This makes the dark side taller than the light side. The shorter well-lit side then pulls on the taller dark side and makes the stem bend towards the light. When this happens both sides are facing the light and both sides make the same amount of growth substance. This means that they both grow at the same speed in the direction of the light.

Do all plants need the same amount of light?
No. Some plants can survive in dim light while others need bright light. In a rainforest, the trees make a huge leafy roof over the ground. This keeps out most of the light but some plants still manage to grow there. They have a purple colour under their leaves. When the dim light passes into the leaf, it reaches the purple layer and is reflected. This means that the green substance gets light from above and reflected light from below so it has enough light to make food. Ferns and mosses are plants which can grow in shady places without the need for a purple layer. They can make food using the energy in the dim light that reaches them.

Can a plant have too much light?
Yes. The holly is an evergreen tree, which grows in Britain. During the winter there may be snow on the ground for a short time. The snow reflects light onto the holly but this does not cause any damage. In the eastern United States snow lies on the ground for much longer and the reflection of light from the snow causes scorching which kills the tree.

Does all the oxygen we breathe come from all the plants around us?
Some of it does but most of it comes from algae in the oceans. Over two thirds of the Earth is covered by seas and oceans and each one contains billions of algae floating in its upper waters. The algae use sunlight to make food and oxygen is released during the food-making process. It provides oxygen for ocean life and some of the oxygen escapes from the ocean surface and mixes with the air.