teins can be studied only after
removing the other materials associated with them in the tissue, by
suitable mechanical or chemical means.

THE SYNTHESIS OF PROTEINS IN PLANTS

The synthesis of proteins in plants is not a process of photosynthesis, as
it can take place in the dark and in the absence of chlorophyll, or any
other energy-absorbing pigment. However, protein-formation normally takes
place in conjunction with carbohydrate-formation. The carbon, hydrogen,
and oxygen necessary for protein synthesis are undoubtedly obtained from
carbohydrates. The nitrogen and sulfur come from the salts absorbed from
the soil through the roots and brought to the active cells in the sap.
Atmospheric nitrogen cannot be used by plants for this purpose, except in
the case of certain bacteria and other low plants, notably the bacteria
which live in symbiosis with the legumes in the nodules on the roots of the
host plants. In general, the sulfur must come in the form of sulfates and
the nitrogen in the form of nitrates; although many plants can make use of
ammonia for protein-formation. Presumably, the nitrate nitrogen must be
reduced in the plant to nitrites, and then to ammonia form, in order to
enter the amino-arrangement required for the greater proportion of the
protein nitrogen.

The mechanism by which ammonia nitrogen becomes amino-acids in the plant is
not understood. Artificial syntheses of amino-acids, by the action of
ammonia upon glyoxylic acid and sorbic acid, both of which occur in plants
and may be obtained by the oxidation of simple sugars, have been
accomplished, and it seems probable that similar reactions in the plant
protoplasm may give rise to the various amino-acids which unite together to
form proteins. Nothing is known, however, of the process by which the more
complicated closed-ring amino-acid compounds, such as proline, histidine,
or tryptophane, are synthetized.

The condensation of amino-acids into proteins, or the reverse
decomposition, is very readily accomplished in all living protoplasm, under
the influence of special protein-attacking enzymes, which are almost
universally present in the cytoplasm. These reactions in connection with
the proteins are similar to the easy transformation of sugars to starches,
and _vice versa_, under the action of the corresponding
carbohydrate-attacking enzymes.

PHYSIOLOGICAL USES OF PROTEINS

There can be no doubt that