176 



Although Winogradsky showed that the nitrogen-fixing power of 

 this soil organism, growing in sugar solutions, is very considerable, wt 

 are quite in the dark as to the extent to which fixation of nitrogen 

 goes on in the soil — whether it is even in excess of the amount lost by 

 drainage, &c. The greatest gain would presumably be in soils contain- 

 ing plenty of organic matter and on uncultivated and undrained land ; 

 the smallest gain if any, would be on ordinary arable land. 



Whilst, according to Berthelot, there are a number of soil organisms 

 which fix nitrogen, Winogradsky found only the one referred to which 

 possesses this power ; and, although he isolated fourteen other micro- 

 organisms from the soil, none of these fixed nitrogen to an appreciable 

 extent in absence of combined nitrogen, although two of them showed, 

 perhaps, a very slight gain when grown in presence of a small amount 

 of combined nitrogen. 



3. It is, then, with the third process, fixation in the root nodules 

 — that we have to deal. It is not necessary here to go into the history 

 •of the subject, as that has already been been pretty fully discussed 

 (^). That fixation does take place has been confirmed over 

 and over again. The biology of the subject has also received a good 

 deal of attention, important results having been obtained by Marshall 

 Ward — who made clear the process by which the micro-organism invades 

 and enters the root tissues — by Tschirch, Brunchorst, and others. And 

 the relation between the plant and its nodules has, to some extent, been 

 explained by the Rothamsted experiments, in which the growth and 

 ripening of the plant were shown to be coincident with progressive ex- 

 haustion of the nitrogenous matter of the nodules whilst on the other 

 hand, with plants not ripe, or of more than annual growth, there was 

 accumulation in the nodules — that is, provision for future growth. (^) 

 It must, however, be admitted that we are still completely in the dark 

 as to the nature of the chemical process by which nitrogen is first 

 brought into combination within the nodules, and what is the first com- 

 pound formed. 



As far as agricultural plants are concerned, it is only those of the 

 leguminous sub-order Papilionace se on the roots of which nodules occur, 

 and which in consequence can thrive without external application of 

 combined nitrogen. It may, however, be mentioned that the symbosis 

 prevails in the case of other leguminous plants — for instance, of the 

 sub-orders Mimoseae and Caesalpinieae, ('') and even of non-leguminous 

 plants such as the common alder. (^) 



The methods employed in pot experiments for inducing the forma- 

 tion of nodules have been the application of a soil extract (Hellriegel), 

 of soil itself (Frank), or the inoculation has been made directly from 

 the contents of a nodule. Hellriegel's earliest results showed that all 



5 Lawes and Gilbert, Journal R.A.S E., 1891. [3] Vol. 11., p. 657. 



6 Lawes and Gilbert, loc. cit. Also Gilbert, U.S. Agric. Dept. Office of Experi- 

 ment Stations, Bui. No. 22, p.l36, et Beq. 



7 Morck Ueber die Formen d. Bakteroiden b. d. oinzelnen Spezies d. Legumi 

 nosen." Inaug. Diss. Leipzig, 1891. 



8 L. Hiltner, " Ueber d. Bedeutung d. Wurzelknollchen v. Alnus glutinosa f.d. 

 Stickstoflfernahrung dieser Pflanze, " Landw. Versuchs-Stat., 1895, Bd. 46, p. 153, 

 and R. Dinger, De els als stikstofverzamelaar. Landbouwnkundig Tijdschrift, 

 J896, p. 167. 



