Nitrogen Nutrition of Green Plants.—It is the belief of botanists that green plants obtain their nitrogen chiefly in the form of nitrates, though ammonium salts may be utilized to some extent by certain plants at least. Exceptions to this general rule are those plants provided with root tubercles (and the bog plants and others which have mycorrhiza?). These plants obtain their nitrogen in the form of organic compounds made for them by the bacteria growing in the tubercles. That nitrogen circulates throughout the structure of plants in organic combination is certain. There does not appear to be any reason why similar compounds which are soluble and diffusible (amino-acids?) should not be taken up through the roots of plants and utilized as such. It seems to the author that this is very probably the case. Arguments in favor of this view are: (1) The nitrogen nutrition of leguminous and other plants with root nodules. (2) The close symbiosis between “Azotobacter” and similar nitrogen-absorbing bacteria and many species of algæ in sea water at least. (3) The vigorous growth of plants in soils very rich in organic matter, which inhibits the production of nitrates by the nitrous-nitric bacteria when grown in culture, and possibly (?) in the soil, so that nitrates may not account for the vigorous growth. (4) The effect of nitrate fertilizers is to add an amount of nitrogen to the crop much in excess of the amount added as nitrate. (5) The most fertile soils contain the largest numbers of bacteria. The doctrine that nitrates furnish the only nitrogen to plants was established before the activities of bacteria in the soil were suspected, and, so far as the author is aware, has not been supported by experiments under conditions rigidly controlled as to sterility.
It would seem that one of the chief functions of soil bacteria is to prepare soluble organic compounds of nitrogen for the use of green plants and thus to make a “short cut” in the nitrogen cycle ([p. 107]), as now believed in, direct from the “decomposition bacteria” to green plants.
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: