338 The Physiology of Plants bookiii 



most important discoveries of the century, that of chemo- 

 synthesis, the consideration of which must be deferred till 

 later. It had been established by Warington that the nutri- 

 tion and growth of the nitrifying organism is injured rather 

 than promoted by good organic nutrients. Starting with 

 this observation, Winogradsky cultivated them in a solution 

 containing only ammonium sulphate, potassium phosphate, 

 and basic magnesium carbonate, and found that when such 

 a solution, properly sterilized, was seeded with a drop of 

 an old culture of the microbe, a strong nitrate reaction was 

 obtained in a few days and in a fortnight all the ammonia 

 contained in the flask was transformed. From the scum 

 which formed on the surface of the culture Winogradsky 

 isolated five micro-organisms which had no nitrifying power. 

 There remained a sediment consisting of a bacterial zoo- 

 gloea; from this he obtained with much difficulty other 

 organisms which proved to be those of which he was in 

 search. Cultures inoculated with this zoogloea gave rise 

 to vigorous nitrification, the more readily if ammonia was 

 added in small quantities and at once replaced when used 

 up. In the culture solutions, however, he found, as did 

 Warington before him, that both nitrites and nitrates 

 appeared in variable quantity. Further experiments showed 

 him what Warington had failed to find, that the formation 

 of nitrite takes place first, and that only after the ammonia 

 is used up does the formation of nitrate from nitrite take 

 place. 



He then attacked the problem with which Warington 

 had grappled with only partial success, viz. to determine 

 whether two organisms are concerned, or whether the whole 

 process is due to one which becomes modified physiologically 

 as it proceeds, and in the course of a series of experiments 

 he obtained conclusive evidence of the existence of two 

 kinds of nitro-bacteria, one of which constructs nitrites 

 from ammonia, the second oxidizes the nitrites to nitrates. 



