part 14

130 The material which makes plants green (chlorophyll) is necessary if they are to use energy from light for healthy growth. Unlike animals, plants can take what they need straight from the air and earth and water in which they live. Through their green leaves and stems they separate H from H2O and unite it with CO2 to make carbohydrates. Structure of part of a carbohydrate, a sugar. This power of plants is named photosynthesis (from the Greek words for light and putting together). People and animals depend upon photosynthesis in plants to supply their food and energy. We and the animals either eat plants or eat plant-eating animals or both.

131 When we think it out we can see that most of the energy we use comes from the sun by way of the materials which plants build up in photosynthesis. Scientists know only a little, so far, about photosynthesis. It is only a little more than a hundred years since the first account of it was given by the German scientist von Mayer. But with more experimenting being done every year, much more will certainly be known before very long. The rate at which scientific knowledge is increasing is going up all the time. In time (but we may have to wait a long time before this) it may even be possible to use the energy of light itself in industry to do in our factories something like what plants do in their green cells.

132 Before long, we may learn from working on photosynthesis how to make or grow more food. As we have seen (page 13 above) more than one billion people on this planet are short of food. As more and more people are born, the need for new food supplies will increase. It may be that we can find what we want in the sea. It is thought that as much as 90 per cent of the photosynthesis which takes place in the world goes on in microscopic water plants in the sea. Maybe much of the food people are going to need in the future will come out of the oceans. Or making food straight from complexes of carbon, hydrogen and oxygen may become a great new industry. At present we have to get most of the energy used in industry by burning coal and oil and other fuels.

133 This energy is the sun's energy stored up long ago by plants. Coal comes from the dead bodies of plants stored, away from the air and under pressure, through millions of years in great beds in the earth. The amount of coal and oil in the earth is, it is true, very great, but the amount we are using today is great too. A time may come before long when we will have used up all the coal and oil that is easy to get at. We have been taking oil out of the earth only since about 1860. Without oil there would be no cars and no airplanes. It is strange to think that airplanes depend on what plants were doing millions of years ago.

134 Plants and animals all need air. Only about one fifth (1⁄5) of the air is oxygen. The other four fifths is another gas, without taste, smell or color, named nitrogen. Both plants and animals need nitrogen as well as oxygen but they cannot take it from the air themselves. How do they get their supply of nitrogen? There are very small organisms in soil and water which can take nitrogen out of the air and fix it in complex materials which plants and animals can use. These materials are taken up by plants through their roots in water from the soil. Animals get their nitrogen by eating plants. Some of this nitrogen comes away in animal waste and people have for many thousands of years known enough to put animal waste back into the soil to help plants grow.

135 Nitrogen, like carbon, oxygen and hydrogen, is used over and over again in support of life. Plants build nitrogen up into complex structures. Animals eat the plants. Then animal waste and dead plants and fallen leaves are broken down into simpler forms again. So the great round goes on without end, all made possible only by energy coming from the sun. Scientists say that all the oxygen and all the carbon dioxide now in the air have been put there after use by plants through photosynthesis.