175 



frequently bear another interpretation, or, if not, are refuted by the 

 OTerwlielming amount of evidence against such an assumption furnish- 

 ed by exact experiments, as well as by general experience in agricultu- 

 ral practice. And as even yet the experiments made between forty and 

 fifty years ago by Ville are sometimes quoted as being the first to prove 

 that fixation of nitrogen takes place during the growth of plants, it 

 may be mentioned that some, at any rate, of his results, which showed 

 an enormous gain of nitrogen, are now known to be incorrect. The 

 conditions under which the experiments were made were such as to 

 exclude the action of micro-organisms which are now known to be 

 essential ; and fixation of nitrogen by non-leguminous plants in absence 

 of any combined nitrogen is disclaimed even by Frank, who, although 

 he believes that all plants have the power of fixing nitrogen, considers 

 the presence of some combined nitrogen necessary to enable non-legu- 

 minous plants to make a start. 



At the present time we are acquainted with three processes by which 

 free nitrogen may be brought into combination, and thus rendered 

 available to plants : (1) By atmospheric electricity, (2) by a micro-organ- 

 ism in the soil, (3) by the symbiosis of certain plants and micro-or- 

 ganisms. 



1. The amount of nitrogen brought into combination by electricity 

 is very limited, and quite inadequate to the wants of any crop. As it 

 is, moreover, equally available for all crops it need not be specially con- 

 sidered here. 



2. As regards fixation in the soil, we include fixation by the micro- 

 organism (Clostridium Pasteurianum) isolated by Winogradsky, (^) and 

 fixation by algae (Schloesing and Laurent), (^) as it seems likely that 

 we have to do with one and the same process. For it must be borne in 

 mind that in Schloesing and Laurent's experiments the algae contained 

 bacteria, and although it is not impossible that Nostoc and some other 

 algae can fix nitrogen, there is, so far, no evidence that algae, in the 

 absence of micro-organisms, have that power. Kossowitsch's experi- 

 ments (*) with different algae, with and without bacteria, favour the 

 view that it is the bacteria which fix nitrogen when supplied with suita- 

 ble carbonaceous matter, such as sugar ; and that certain gelatinous algae, 

 e.g. Nostoc, take the place of sugar in furnishing the bacteria with 

 carbon. This would account for the fact that some soils, when exposed 

 to light, gain nitrogen, whilst in absence of light they do not. In the 

 first case there is a development of algae, in the second case no algae 

 are formed ; and without algae to supply carbon for the bacteria, fixa- 

 tion of nitrogen cannot take place. It is, however, doubtful whether, 

 as has been suggested, we have here to do with a true symbosis, as in the 

 case of root nodules, which are special organs formed on the roots of the 

 plant under the influence of the bacterium, and in which the bacte- 

 rium itseK undergoes modification. Ihis change in the form of the 

 micro-organism seems to be essential ; no appreciable fixation has, at 

 any rate, been observed in pure cultures of the unmodified form. 



2 Archives des Sciences Biol, 1895, tome 111., p. 279, et. seq. 



3 Ann. Inst. Pasteur, tome vi , p. 824. Also Fream, " Fixation of Free Kitzo- 

 gen by the Lower Green Plants." Journal R.A.S.E , 1892, [3] Vol. Ill p. 427. 



4 Bot. Zeitung, 1894, Bd. 52, p. 97. 



