We well know that marine plants cannot derive a supply of humus for their nourishment through their roots.  Look at the great sea-tang, the Fucus giganteus:  this plant, according to Cook, reaches a height of 360 feet, and a single specimen, with its immense ramifications, nourishes thousands of marine animals, yet its root is a small body, no larger than the fist.  What nourishment can this draw from a naked rock, upon the surface of which there is no perceptible change?  It is quite obvious that these plants require only a hold,—­a fastening to prevent a change of place,—­as a counterpoise to their specific gravity, which is less than that of the medium in which they float.  That medium provides the necessary nourishment, and presents it to the surface of every part of the plant.  Sea-water contains not only carbonic acid and ammonia, but the alkaline and earthy phosphates and carbonates required by these plants for their growth, and which we always find as constant constituents of their ashes.

All experience demonstrates that the conditions of the existence of marine plants are the same which are essential to terrestrial plants.  But the latter do not live like sea-plants, in a medium which contains all their elements and surrounds with appropriate nourishment every part of their organs; on the contrary, they require two media, of which one, namely the soil, contains those essential elements which are absent from the medium surrounding them, i.e. the atmosphere.

Is it possible that we could ever be in doubt respecting the office which the soil and its component parts subserve in the existence and growth of vegetables?—­that there should have been a time when the mineral elements of plants were not regarded as absolutely essential to their vitality?  Has not the same circulation been observed on the surface of the earth which we have just contemplated in the ocean,—­the same incessant change, disturbance and restitution of equilibrium?

Experience in agriculture shows that the production of vegetables on a given surface increases with the supply of certain matters, originally parts of the soil which had been taken up from it by plants—­the excrements of man and animals.  These are nothing more than matters derived from vegetable food, which in the vital processes of animals, or after their death, assume again the form under which they originally existed, as parts of the soil.  Now, we know that the atmosphere contains none of these substances, and therefore can replace none; and we know that their removal from a soil destroys its fertility, which may be restored and increased by a new supply.