128

Scientific Memoirs by

Pasteur's work.

The rôle of oxy-
gen.

   the stage of every living organism, and although we cannot say of the vegetable
cell that it is concerned exclusively with synthetic functions, nor of the animal
cell that it maintains its vital energy solely by decomposition, yet we recognize
that, broadly, vegetable and animal mutually subserve each other's needs in the
rhythmical flow of energy which never varies in its final amount. Inasmuch
as the animal cell is unable to complete the resolution of the complex potential
compound into its final kinetic terms so as to be available for plant food, we need
a third kind of organism, half plant and half animal, to complete the cycle, and
whose metabolic relations are not so sharply differentiated as those of the higher
organised plants and animals. This link is supplied by the fungi, which
obtain their necessary Carbon and Nitrogen from more or less complex chemical
compounds, organic (and, for the most part, dead) and inorganic; they
thus subserve both plant and animal life, by rendering the food elements available,
and by the transmution of dangerous waste products. For this they are fitted by
the wide limits within which their external vital conditions can vary, and by their
rapid multiplication, for which they use up a large amount of material in a short
time, a small portion of which goes to provide for their own growth, while a
far larger proportion is decomposed by their fermentative actions and is thus
prepared for final oxidation. One of the first steps in our knowledge of their
physiology was the discovery by Pasteur that the yeast and mould fungi were, in
so far, like the higher plants, that they could assimilate their necessary Nitrogen
and Carbon from ammoniacal salts and even from nitrates, and thus these were
differentiated from the great class of fission fungi, which in their vital require-
ments stand nearer to the animal organism. It was further shown by the same
observer that micro-organisms exhibited the most marked differences in their relation
to free oxygen which had been supposed to be a sine quâ non for all living beings.
One great class requires free oxygen and leads to rapid oxidation of their medium,
while another can not only live without the free element, but can only fulfil their
proper functions in its absence, and that, given this condition, an extensive but
superficial decomposition of the nutrient material takes place, which represents
the fundamental dynamic phenomenon of fermentation. It is true that Pasteur's
dictum, "fermentation is life without oxygen," has, on account of later knowledge,
lost the force and truth of a universal law, and, moreover, we have learnt that the
majority of putrefactive organisms belong to the class of facultative anaërobes,
and that their vital functions vary in the presence or absence of free oxygen,*
much as the products of a smouldering kiln and a blast furnace vary. A microbe
that for its own vital advantage would better thrive in free oxygen, will, if

* See Duclaux, " Traite de Microbiologie," Tome I.