Section I.—Form in Which the Carbon, Etc. Enter Into Plants.

It is from their food that plants derive the carbon, hydrogen, oxygen, and nitrogen, of which their organic part consists. This food enters partly by the minute pores of their roots, and partly by those which exist in the green part of the leaf and of the young twig. The roots bring up food from the soil, the leaves take it in directly from the air.

Now, as the pores in the roots and leaves are very minute, carbon (charcoal) cannot enter into either in a solid state; and as it does not dissolve in water, it cannot, in the state of simple carbon, be any part of the food of plants. Again, hydrogen gas neither exists in the air nor usually in the soil—so that, although hydrogen is always found in the substance of plants, it does not enter them in the state of the gas above described. Oxygen exists in the air, and is directly absorbed both by the leaves and by the roots of plants; while nitrogen, though it forms a large part of the atmosphere, is not supposed to enter directly into plants in any considerable quantity.

The whole of the carbon and hydrogen, and the greater part of the oxygen and nitrogen also, enter into plants in a state of chemical combination with other substances; the carbon chiefly in the state of carbonic acid, and of certain other soluble compounds which exist in the soil; the hydrogen and oxygen in the form of water: and the nitrogen in those of ammonia or nitric acid. It will be necessary therefore briefly to describe these several compounds.