Experiments have been made by different observers in growing seedling plants of various kinds in water culture with one or in some cases several of the amino-acids as sources of nitrogen. Most of these experiments were disappointing. Plant proteins are not so different from animal proteins, or plant protoplasm (apart from the chlorophyl portions of plants) from animal protoplasm as to lead one to suppose that it could be built up from one or two amino-acids any more than animal protoplasm can. The author is strongly convinced that this subject should be thoroughly investigated. It will require careful experimentation and perhaps rather large funds to provide the amounts of amino-acids that would probably be needed, but might result in a decided change in our ideas of soil fertility, and especially in the use of nitrogen fertilizers. CHAPTER XII. PHYSIOLOGICAL ACTIVITIES (CONTINUED). PRODUCTION OF ENZYMES. Most of the physiological activities of bacteria which have been discussed are due to the action of these peculiar substances, so that a knowledge of their properties is essential. This knowledge cannot as yet be exact because no enzyme has, up to the present, been obtained in a “pure state,” though it must be admitted that there are no certain criteria which will enable this “pure state” to be recognized. It was formerly thought that they were protein in nature, but very “pure” and active enzymes have been prepared which did not give the characteristic protein reactions, so this idea must be abandoned. That they are large moleculed colloidal substances closely related to the proteins in many respects must still be maintained. There are certain characteristics which belong to enzymes, though no one of them exclusively. These may be enumerated as follows: 1. Enzymes are _dead_ organic chemical substances. _Dead_ is used in the sense of non-living, never having lived, not in the sense of “ceased to be alive.” 2. They are always produced by _living_ cells: Sometimes as active enzymes, sometimes as _pro-enzymes_ or _zymogens_ which are converted into enzymes outside the cell by acids, other inorganic substances or other enzymes. 3. They produce very great chemical changes without themselves being appreciably affected. Enzymes will not continue to act indefinitely, but are used up in the process (combination with products?). The amount of change is so great in proportion to the amount of enzyme that the above statement is justified in the relative sense. Thus a milk-curdling enzyme has been prepared that would precipitate 100,000,000 times its own weight of caseinogen. 4. Their action is specific in that each enzyme acts on one kind of chemical substance only, and the products are always the same.