Even in one of the least agricultural of all civilized countries, such as Great Britain, the cultivation of plants is still the largest national industry. In 1897 we grew enough corn to give a ration of 1lb. per diem to every inhabitant for 68 days, and we manage to get a large amount from every acre (28 to 33 bushels per acre). In most other countries the relative importance of land and of agriculture generally is very much greater than it is in Britain. Moreover, it seems at first sight as if all this harvest had been made out of nothing at all. Plants do take in a small amount of mineral matter from the earth, but these minerals form but a very little part of the bulk of a tree or any vegetable substance. A piece of wood can be burnt up in a fire and very little indeed of it is left. A few ashes will indeed remain, which are the minerals taken in from the earth, but all the rest has vanished into the atmosphere. The water which was contained in the wood has become steam and is evaporated; the woody matter consisted chiefly of compounds of a chemical substance, carbon, which also becomes an invisible gas (carbonic acid gas) in a fire and goes back into the atmosphere. When the piece of wood was formed in a growing tree, it is easy to see where the water came from: it was taken in by the roots. Just as flowers drink up the water in a vase, and wither if they do not receive enough, so all plants suck up water by their roots. The carbonic acid gas is taken into plants through their leaves and is worked up into sugar, starch, wood, and other matters inside the plant. But there is another very interesting point about the way in which wood is burnt in a fire; heat and light are obtained from a wood fire. Where did that heat and light come from? If you walk in summer, under a tree in full leaf, it is much cooler than it is in the sunshine outside. This shows what happens: the sunshine has been taken up or absorbed by the leaves of the tree. It does not pass through the foliage, but the heat and light are stopped by the leaves. The light and heat which were used up by the leaves in making wood, sugar, and starch come back again when that wood or starch is burnt. So that the burning up of a bit of wood is just the opposite to the formation of that wood in sunshine in a living tree. The important point is that it is the sunshine which is used by plants to make all these refractory bodies, such as water, carbonic acid gas, and others, unite together to form sugar, starch, and wood.