As a plant grows, water from the soil or air unites chemically with carbon, and forms the woody fibre of the stem, the sugar of the sap, and the starch of the seed. When the plant dies, the water is again set free from its structure and passes into the air. The starch and sugar, also, which the plant yields having been consumed by some animal, the water which they contain passes into the air through the lungs and skin. Thus the same water is caused to revolve in a circle of life-sustaining combinations. Within a single hour it may be in some vegetable structure in the form of sugar; then it may pass into and circulate through some animal system and be discharged as vapor from the lungs, and afterwards become absorbed by thirsty leaves and aid in the growth of flowers and fruits.
1 This article is mainly an abstract of the chapters on the "Circulation of Matter," contained in "Johnston's Chemistry of Common Life." It is inserted here as bearing upon the general subject of food, and with the hope that all who read this will read the original.