plant can raise the less complex substances--carbonic acid, water, and
nitrogenous salts--to the same stage of living protoplasm, if not to the
same level. But the plant also has its limitations. Some of the fungi,
for example, appear to need higher compounds to start with; and no known
plant can live upon the uncompounded elements of protoplasm. A plant
supplied with pure carbon, hydrogen, oxygen, and nitrogen, phosphorus,
sulphur, and the like, would as infallibly die as the animal in his bath
of smelling-salts, though it would be surrounded by all the constituents
of protoplasm. Nor, indeed, need the process of simplification of
vegetable food be carried so far as this, in order to arrive at the
limit of the plant's thaumaturgy. Let water, carbonic acid, and all the
other needful constituents be supplied except nitrogenous salts, and an
ordinary plant will still be unable to manufacture protoplasm.

Thus the matter of life, so far as we know it (and we have no right to
speculate on any other), breaks up, in consequence of that continual
death which is the condition of its manifesting vitality, into carbonic
acid, water, and nitrogenous compounds, which certainly possess no
properties but those of ordinary matter. And out of these same forms of
ordinary matter, and from none which are simpler, the vegetable world
builds up all the protoplasm which keeps the animal world a-going.
Plants are the accumulators of the power which animals distribute and
disperse.

But it will be observed, that the existence of the matter of life
depends on the pre-existence of certain compounds; namely, carbonic
acid, water, and certain nitrogenous bodies. Withdraw any one of these
three from the world, and all vital phenomena come to an end. They are
as necessary to the protoplasm of the plant, as the protoplasm of the
plant is to that of the animal. Carbon, hydrogen, oxygen, and nitrogen