336 The Physiology of Plants BOOK in 



determining factor and that the influence of the external 

 conditions was at any rate subordinate. He said, ' It is 

 indeed possible to have two similar solutions under identical 

 external conditions, in one of which only nitrites and in 

 the other only nitrates are being produced ; the difference 

 being determined by the character of the organisms with 

 which the respective solutions have been seeded.' After 

 giving further proofs of nitrification being due to an organ- 

 ism, and claiming that the idea was completely established, 

 he went on to describe the distribution of the organism in 

 the soil and to show the depth to which it penetrates. 



In the course of his memoir of this date Warington said 

 that in 1879-80 he was able to prepare a culture solution 

 (of course containing microbes), which was incapable of 

 oxidizing ammonia, but was energetic in converting nitrites 

 into nitrates. He held at the time the opinion that this 

 course of action indicated merely a phase in the life history 

 of the nitrifying organism. 



In a further paper, published in 1891, he gave special 

 attention to the conditions under which nitrites are formed, 

 and those under which they are reduced to or replaced by 

 nitrates. He started with the observation that in natural 

 soil, in .ordinary cases, the nitrites are due to the action 

 of reducing bacteria, working in a deficient supply of 

 oxygen, but that this is not the case when a strong 

 solution of some ammonium salt is treated with a small 

 quantity of soil. Oxidation then takes place, first nitrites, 

 and later nitrates being formed. Then, reverting to the 

 experiments that led him to the conclusion in 1884 already 

 quoted, he described how he had taken one of the cultures 

 that contained only nitrites (in which he had said the 

 nitrifying agent had ' assumed the nitrous character ') and 

 continued its cultivation for three years (1879-82), keeping 

 it pure. It never formed nitrates under any conditions. 



He again interpreted his result by ascribing the behaviour 



