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38 GENERAL AND PHYSICO-CHEMICAL.
are only called upon to appropriate and assimilate material already exist-
ing and to transform it into new forms of energy.
Out of a few relatively simple combinations, especially carbon
dioxide and water, together with ammonium compounds or nitrates,
and a few mineral substances, which serve as its food, the plant builds
up the extremely complicated constituents of its organism—proteins,
carbohydrates, fats, resins, organic acids, etc. The chemical work
which is performed in the plant must, therefore, in the majority of cases,
consist in syntheses; but besides these, processes of reduction take
place to a great extent. The radiant energy of the sunlight induces
the green parts of the plant to split off oxygen from the carbon dioxide
and water and this reduction is generally considered as the starting-
point in the syntheses that follow. According to a hypothesis suggested
by A. Baeyer,1
formaldehyde is first produced, C02+H20 = CH20+02,
which by condensation is transformed into sugar. From the sugar other
bodies can then be built up.
With the aid of the silent electric discharge W. Loeb 2
has succeeded
in obtaining from carbon dioxide and water, formaldehyde, and as a
product of polymerization, also glycolaldehyde, CH2OH.CHO, from
which sugar can be readily produced. Still the conditions under which
these bodies were formed cannot be applied to the conditions in the
plants. The investigations of Usher and Pristley 3
are of greater
interest in that they show the formation of formaldehyde in the photo-
lytic decomposition of moist carbonic acid in the presence of chloro-
phyll. These investigations also do not seem to be entirely free from
exception. The conception as to the formation of sugar from formalde-
hyde is also often different from that explained by v. Baeyer’s idea,
and his view as to the assimilation of carbonic acid constitutes a hypoth-
esis which requires further proof. The essentials of this hypothesis,
namely, a formation of formaldehyde with a subsequent sugar formation
by condensation of the aldehyde groups, is still very generally accepted
as probably correct. Independent of the ways and means of how the
assimilation processes in the plants originate, it is obvious that the free,
radiant energy of the sun is hereby bound and stored in a new form, as
chemical energy, in the combinations formed by the syntheses.
In animal life the conditions are not the same. Animals are depend-
ent either directly, as the herbivora, or indirectly, as the carnivora,
upon plant-life, from which they derive the three chief groups of organic
nutritive matter—proteins, carbohydrates, and fats. These bodies,
of which the protein substances and fats form the chief mass of the
1
Ber. d. d. chem. Gesellseh., 3.
*Zeitschr. f. Electrochem., 12.
1
Proc. Roy. Soe. London, 78, Series B.
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