CHAP, in The Absorption of Nitrogen 329 



plied with air free from compounds of nitrogen. He found 

 they would not grow till a trace of nitrate was mixed with 

 their nutritive medium, and then only in proportion to 

 the amount of nitrate so supplied. Then he used cultures 

 of algae, together with some of the soil bacteria, with no 

 organic compounds. A small increase of the combined 

 nitrogen was observed. Later on cultures of mixtures of 

 several algae, including Nostoc, together with bacteria, 

 resulted in a considerable fixation of nitrogen. In all his 

 series he found the fixation much larger in the presence 

 of sugar. 



Kossowitsch concluded that the association of the bacteria 

 with the algae, in a kind of symbiotic union, is the cause, 

 or condition, of the fixation. In this union the algae, 

 supplied with nitrogen by the bacteria, form or construct 

 carbohydrate material, part of which is given to the nutri- 

 tion of the microbes. This conclusion is supported by the 

 observations that if the mixed culture is placed in the 

 light there is a larger fixation than in darkness, and that 

 if abundant carbon dioxide is supplied still more nitrogen 

 is fixed. 



The species of algae which have a mucilaginous envelope 

 wherein the microbes become enveloped, carry out the 

 process of fixation best, and the addition of sugar to such 

 is attended with less effect than it is to forms whose walls 

 are not mucilaginous. 



The determination of the non-green member of the sym- 

 biosis remained to be made. In 1895 Puriewitsch claimed to 

 have shown that the mould iungiAspergillus and Penicillium 

 are sometimes concerned, but the evidence does not seem 

 conclusive. In the same year, however, light was thrown 

 upon the point by the researches of Winogradsky. He 

 inoculated with arable soil a culture fluid containing no 

 combined nitrogen. Very speedily there was apparent a 

 vigorous butyric-acid fermentation and the organisms con- 



