The vegetable world sustains a fundamental relation to man's physical being. Plant life is the medium through which the inorganic substances of the soil and air are made available to the uses of the human organism; and so it is with the materials and activities of plants that we must begin a study of the basal facts of human nutrition.

1. The Plant Stores Food Substance

The first step toward supplying man with food is taken when the farmer drops seed into the warm earth. As soon as the young rootlets from a germinating seed come in contact with the soil, and the first leaves reach the air, assimilative growth begins. During the hours of sunlight, matter is constantly gathered in an invisible way, which, after transformation into various compounds, is added to the enlarging tissues of the plant. This continues, perhaps for a season, until the stalk of grain has reached its full height and has attained the ultimate object of its existence in the production of seed. The farmer carries to the field a few pounds of seed, and he returns to his storehouses laden with tons of new material, perhaps hay, perhaps grain. From somewhere, in some way, the plant has gathered various substances, often no less than ten thousand pound per acre in a single year, and has manufactured them into forms that are nutritively useful to man.

2. The Plant Stores Energy

Plant life not only builds tissue: it stores energy, as we may easily discover. The farmer's boy learns this when he feels the hot glow of the fire that is fed by forest wood. The wood disappears, but he is warmed by the radiant heat. It has occurred that when fuel was scarce and costly and grain was abundant and cheap, the Western farmer has burned his corn. As with the wood, the materials which were collected from the soil and air were dispersed in invisible forms during the combustion which liberated the heat energy, except a small heap of ashes on the hearth.

3. Relation Of Plant Substance To Animal Life

But ordinarily, the substance of farm crops is produced, not for fuel but for food purposes, and in this use of vegetable matter we come in contact with a set of phenomena equally complex and equally important and interesting to those of its growth. The child gradually attains a man's stature. What is the source of this added tissue? It is plant substance which in other combinations was collected from soil and air. The child eats his daily food and makes his daily gain of tissue. If his food were withdrawn, his body would waste, and in time death would ensue. We, therefore, cannot resist the conclusion that the bones, blood, and flesh of the human body are derived from food.

The plant does more than to supply building material for the animal1 body, for the living organism is kept warm. No matter how cold the surrounding atmosphere, we find by the use of a thermometer that in health man's temperature remains at about 98.4° F. with not over one degree normal variation. Just as the Western farmer obtained heat by burning corn in the fireplace, so do human beings maintain their body temperature at the necessary degree by consuming food to be burned. The combustion is not so rapid as occurs in the fireplace, but the chemical changes are the same, only more slowly carried on.

1 Man is an animal, and the nutritive processes of his body are similar to those in all other higher animal organisms,the mechanism being different.

Food not only builds man's body and warms it, - it furnishes it with motive power. The energy which the plant acquires during its time of growth through vital processes is transformed in part into motion. Man is a living mechanism, a combination of muscles and levers which are moved, not by means of a spontaneous internal generation of energy, but through a supply from without, the energy stored in the plant.

If we use the plant for fuel, we get heat alone; if it is consumed as food, we get heat, motion and the production of body tissue. In the first instance, the plant substance, except the mineral portion, is wholly broken up into simpler compounds which escape in unseen gaseous forms, the liberated energy becoming manifest as heat. If the plant is used by man as food, a greatly varying proportion of its dry matter is retained to form his body substance, and the remaining part suffers oxidation, largely into the same compounds that are carried away by the draft from the fire on the hearth, with an accompanying liberation of energy that manifests itself in motion and heat. As a result, there is built a living organism that is warmed to a temperature generally much above that of the surrounding air, and which is the seat of complex internal activities and is capable of performing external work.