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 "conjugated proteins" in particular, their structure is very complex. The molecular weight of some of the proteins has been determined directly, in the case of those particular ones which can be prepared in proper form for the usual determination of molecular weight by the osmotic pressure method; and has been computed for various others, from the percentage of sulfur found on analysis, or (in the case of the hæmoglobin of the blood) from the proportion by weight of oxygen absorbed. From these determinations and computations, the following formulas for certain typical proteins have been calculated: for zein (from Indian corn), C_{736}H_{1161}N_{184}O_{208}S_{3}; for gliadin (from wheat), C_{685}H_{1068}N_{196}O_{211}S_{5}; for casein (from milk), C_{708}H_{1130}N_{180}O_{224}S_{4}P_{4}; for egg-albumin, C_{696}H_{1125}N_{175}O_{220}S_{8}. These few examples will serve to illustrate the enormous size and complexity of the protein molecule. The conjugated proteins are still more complex than the simple proteins whose formulas are here presented.