3. Our dependence upon plants. Let us consider for a moment how much we depend upon the vegetable kingdom. Every one knows that in all we eat and drink, the nutritious, strength-giving part comes either from plants or animals. As the animals which yield us food depend in their turn either upon plants or upon plant-eating animals, it follows that if it were not for plants the whole animal kingdom, ourselves included, would soon starve. So too in the matter of clothing we depend partly upon the plants which yield cotton, flax, and similar materials, and partly upon those plant-fed animals which give us silk, wool, and leather. Forests yield the chief materials for ships and other means of transportation, for houses, furniture, and innumerable utensils. The fuel which cooks our food, heats our dwellings, and drives the machinery of factories, ships, and locomotives, comes either from plants recently alive or from coal-plants which died long ages ago and were buried in the earth. In sickness, too, the drugs which allay our suffering and help to cure us, are almost entirely of vegetable origin. So whichever way we turn we find plants serving us in most important ways- feeding us, clothing us, sheltering us, warming us, working for us, and making us well-indeed, our dependence upon them is so constant that we seldom realize how intimately our lives are bound up with theirs.