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AGE2vCY OF 3ITCR0-0R0ANISMS IN CARIES. 757

furnished the means of dissolving it (Sachs supposes this solvent to be
carbon dioxide), and have appropriated such of its elements as were
demanded by the needs of the plant. This is an illustration of the
universal law that all living things, both plant and animal, must digest
and prepare food-material for assimilation. In the physiological sense
it is not essentially different from the digestion which takes place in all
the higher animals, including man. In the higher animals this is a
very complex process performed by an elaborate jjhysical mechanism.
In the walls of the stomach a special tissue is developed, the office
of which is to prepare the unorganized ferment, pepsin, which, with
the aid of other substances elaborated by similarly specialized organs,
performs the office of digestion for the whole group of cellular forms
that constitute the animal.
In the seed there is still a division of labor. Here we find, indeed,
in the perigerm an organ set apart for the accomplishment of the act of
dio'estion as in the hioher animals, but its mechanism is so different and
so simplified that nothing but the closest study of its functions w'ill
remind the student of the analogy that exists between it and the stomach
of the animal. In the plant there is no specialized group of cells for
this office, but the performance of the act is distributed among the root-
lets. Now, when we follow this process down through the lowly organ-
isms, we find a continuous simplification of the mechanism for the per-
formance of this function until all trace of a specialized organ is lost.
Shall we conclude from this that the function is lost? Is it not more
probable that as we descend to those very lowly organisms we will find all
of these functions combined in the single cell ? If, now, we turn our atten-
tion to the plant known as the torula, which is the active agent in alco-
holic fermentation, and study its physiology, we find the following fiicts :
" When pure vinous yeast is washed with distilled water, a peculiar
substance is found dissolved in the water. This is yielded continually
during the life of the plant. Examinations have proven this substance
to be an unorganized ferment having a peculiar effect upon sugar. This
has been examined by Berthelot, Becamp, and others. It has been pre-
cipitated and obtained in the form of a powder somewhat similar to pep-
sin, and when redissolved has been found to retain its original power
over cane-sugar. This action is to split up the sugar into tMO sub-
stances, called glucose and levnlose
"This reaction always takes place as the primary step in alcoholic
fermentation, and is the primary digestion which permits the appropri-
ation of the food-material by the yeast-plant. This is entirely analogous
to the digestion of food in the stomach of an animal, by which such food
is received in the blood, to be conveyed to the tissues for their nutrition,
and is the same as the digestion of starch in the seed ; but it is accom-
plished in the surrounding media instead of a receptacle provided for
the pur]3ose.
" This is one instance of a type of digestion which I believe to be
universal in case of all unicellular animals and plants. The formation of
a stomach is a j^rovision for the conservation of force, but it in no way
changes the modus operandi of the digestive function." ^
^ Formation of Poisons by Micro-organisms, p. 84, Black.