. Plant physiology. Plant physiology. ASSIMILATION OF CAEBON 41 (more properly, just because of this very simplicity) they usually exhibit far-reach- ing physiological peculiarities. Each one of these organisms carries out its own little work, but it constitutes a very important link in the processes of nature. For example, the presence of two kinds of bacteria appears to be requisite for the oxidation into nitric acid of the ammonia present in the soil. One of these (Nitrosomonas) carries the oxidation as far as nitrous acid, the other (Nitro- bacter) oxidizes this to nitric acid. Ammonia is
. Plant physiology. Plant physiology. ASSIMILATION OF CAEBON 41 (more properly, just because of this very simplicity) they usually exhibit far-reach- ing physiological peculiarities. Each one of these organisms carries out its own little work, but it constitutes a very important link in the processes of nature. For example, the presence of two kinds of bacteria appears to be requisite for the oxidation into nitric acid of the ammonia present in the soil. One of these (Nitrosomonas) carries the oxidation as far as nitrous acid, the other (Nitro- bacter) oxidizes this to nitric acid. Ammonia is essential as nutrient material for the first form and nitrous acid is a waste. But this by-product constitutes an essential food substance for the other form. Is it possible, then, to conceive of some nutrient medium that would be equally well suited for the nutrition of both these bacteria? This question must receive a negative answer; a medium must be used that is favorable only to the microorganism under investigation, and that is especially adapted to its particular requirements. The use of such media is highly important if pure cultures are desired. This use has been desig- nated by Vinogradskii as the method of "selective ; A culture is selective if it promotes only a certain func- tion, or if it promotes a function which is as restricted as possible. The more closely limited or exclusive are the conditions, the more favorable will these conditions be for one species possessing a particular property or function, in contrast to others not so endowed, and the growth of these latter in a medium thus alien to them will be quite impossible or at least very difficult. In thus assisting the desired microorgan- isms in their struggle for existence, we in- ^^^ ^ £orms of bacteria. crease their numbers in our cultures and thereby render their discovery easier. When a specific bacterium has once been found it is thus usually possible to discover also the
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