On the Sources of the Nitrogen of Vegetation, 115 



described, that it is impossible to accept the whole as they stand, and 

 for the present it seems necessary to hold judgment on them in. 

 abeyance. 



The various results alluded to will be discussed in some detail in 

 our full paper, but we can only briefly refer here to some of the 

 various modes of explanation which have been suggested. 



In the experiments of M. Berthelot, in all of which the gains of 

 nitrogen are comparatively small, they have in some cases been attri- 

 buted to electrical action, and in others to the action of micro-organ- 

 isms within the soil. 



Frank, experimenting with a soil very rich in nitrogen, found a loss 

 of combined nitrogen ; but, in the case of vegetation experiments, with 

 a less rich soil, he generally found a gain. He concluded that two 

 opposite actions are at work within the soil — one by which nitrogen 

 is set free, and another by which it is brought into combination ; the 

 latter being favoured by the presence of living plants. He admits 

 that there is no decisive evidence how this takes place ; but he seems 

 to assume that it is under the influence of micro-organisms. 



Hellriegel, again, found that lupins did not grow well in an ex- 

 perimental soil, until he added to it the watery extract of a soil from 

 a field where lupins were growing luxuriantly. After this, his ex- 

 perimental plants also grew well, developed the well-known nodules 

 on their roots, and showed a gain of nitrogen. This, he suggested, 

 was probably due to the action of the nodules within the soil, bringing 

 the free nitrogen of the air into combination, and thus rendering it 

 i available to the growing lupins. The results of Tschirch and these of 

 Brunchorst have, however, been held to be conclusive against such a 

 view. According to their experiments, the nodules have no external 

 communication with the soil, but receive their nutriment from the 

 plant itself. On this point it is of interest to observe that, according 

 to the recent experiments of Mr. Marshall Ward, on the death of the 

 nodules the spores become distributed in the soil, and, if this be the 

 case, the possibility of some action, whatever that may be, is not yet 

 disproved. 



Whatever may be the exact facts in the cases cited, it is at any rate 

 clear that recent lines of explanation of the mode in which some of 

 the higher plants derive their nitrogen involve the supposition of the 

 intervention of lower organisms in some way. It must, however, be 

 admitted on a review of the conflicting results at present at command, 

 that they do not justify any confident conclusion that the compensa- 

 tions supposed do take place in any important degree, or that free 

 nitrogen is to any important extent brought into combination under 

 the influence of the lower organisms. In the meantime it seems not 

 inappropriate to devote attention to some other aspects of the subject. 



We would submit that a careful consideration of the history of 



