GEO.--"The Simpler Natural Bases," 215 pages, _Monographs_ on
Biochemistry, London, 1914.

FISCHER, E.--"Untersuchungen in der Puringruppe, 1882-1906," 608 pages,
Berlin, 1907.

HENRY, T. A.--"The Plant Alkaloids," 466 pages, Philadelphia, 1913.

JONES, W.--"The Nucleic Acids," 118 pages, _Monographs_ on Biochemistry,
London, 1914.

PICTET, A.--"La Constitution Chimique des Alcaloides Vegetaux," 421 pages,
Paris, 1897 (2d ed.).

VAUGHAN, V. C. and NOVY, F. G.--"Ptomaines, Leucomaines, Toxins and
Antitoxins," 604 pages, Philadelphia, 1896, (3d ed.).

WINTERSTEIN, E. and TRIER, G.--"Die Alkaloide," 340 pages, Berlin, 1910.

CHAPTER XIII

PROTEINS

The proteins are the most important group of organic components of plants.
They constitute the active material of protoplasm, in which all of the
chemical changes which go to make up the vital phenomena take place.
Combined with the nucleic acids, they comprise the nucleus of the cell,
which is the seat of the power of cell-division and, hence, of the growth
of the organism. Germ-cells are composed almost exclusively of protein
material. Hence, it is not an over-statement to say that proteins furnish
the material in which the vital powers of growth and repair and of
reproduction are located. A recognition of their importance is reflected in
the use of the name "protein," which comes from a Greek word meaning
"pre-eminence," or "of first importance."

In addition to the proteins which constitute the active protoplasm, plants
also contain large amounts of reserve, or stored, proteins, especially in
the seeds. In the early stages of growth, the proteins are present in
largest proportions in the vegetative portions of the plant; but as
maturity approaches, a considerable proportion of the protein material is
transferred to the seeds.

GENERAL COMPOSITION OF PROTEINS

The plant proteins are fairly uniform in their percentage composition. The
analyses of some sixteen different plant proteins show the following
maximum limits of percentages of the different chemical elements which they
contain: Carbon, 50.72-54.29; hydrogen, 6.80-7.03; nitrogen, 15.84-19.03;
oxygen, 20.86-24.29; sulfur, 0.17-1.09. Animal proteins vary more widely,
both in percentage composition and in properties, than do those of plant
origin.

Protein molecules are very large and, in the case of the so-called