All living matter requires for life, food, air, water. Thirst, hunger, suffocation, result from lack of water, food, air. Life ceases when living things are deprived of air. Lack of water can be endured less long than lack of food. But the greater effort required to secure food makes it seem the most important need of life. Air, water, food, are all essential; air continuously, water and food, periodically.

Vegetation supplies plant food and purifies the atmosphere. Its production of plant food is no less significant to human life than its effect upon the air-supply. Plants live as do animals and humanity. All breathe alike in that they take in air and give out carbon dioxid (CO2) both day and night. This process of living is called respiration.

In the other processes of living, plants and animals differ more. But in the internal activities of physical living, animals and humanity are very similar, though their food differs in kind. The food of plants differs from that of both. Plants themselves become the food of both animals and humankind. Foods are of use to living organisms only as they unite with the oxygen of the air breathed in during respiration. This combining of food and oxygen is part of the process of nutrition; that is, the physiological utilization of food.

Green plants use as food the carbon of the carbon dioxid that is breathed out by all living things. The green coloring matter of plants seen in leaves is called chlorophyll. Through the agency of chlorophyll green plants have the power in the presence of sunlight to break up carbon dioxid, use the carbon as food, and return the oxygen it contains to the atmosphere. But for this function of green vegetation the carbon dioxide breathed out by the living of humanity, animals and plants would render the air unable to support life.