1800.] On the Question of the Fixation of Free Nitrogen. 109 



growing lucerne ; and, at the same time, a solution of calcium nitrate 

 was added to pot 3, and the application was continued, as in the case 

 of the clover. For many weeks the repeated soil- extract seeding was 

 without any apparent effect ; but, quite recently, there has been a 

 slightly increased depth of colour, and. perhaps a little growth. The 

 application of nitrate to pot 3, however, showed marked effect very 

 soon after the application had been commenced ; and, as the repre- 

 sentation of the growth on December 23 shows, there was up to 

 that time, considerable growth under the influence of the nitrate. 



The darkening of the colour of the leaves of the clover, and the 

 increased growth of the lucerne, under the influence of the nitrate, in 

 soil otherwise nitrogen-free, is of interest. Not that there is any 

 want of abundant evidence showing that Leguminosse do take up 

 nitrogen largely as nitrate, but, in view of the new results under the 

 influence of micro-organism seeding, it seems to be assumed by some 

 that these plants probably depend for their nitrogen exclusively on 

 such agency. 



Before concluding in regard to the experimental plants, some 

 reference should be made to the very great difference in the external 

 appearance and character of the swellings, or nodules, on the roots of 

 the different descriptions of plant, and even on those of the same 

 description. In the course of the examinations this was so marked, 

 that it was contemplated to take photographs illustrating the most 

 characteristic differences ; and, as it was found that the roots of the 

 experimental plants, which had to be preserved for analysis, could 

 not without risk be manipulated as required for the purpose, some 

 plants were procured from the garden and the fields, and notes of 

 previous observations were looked up. Presumably owing to the late 

 period of the season, the roots so obtained were, however, not suitable 

 for the illustration desired. It must suffice, therefore, avoiding any 

 attempt at technical description, to make a few general observations 

 on the facts at command ; and to say that we hope to follow the 

 subject up at a more suitable season of the year, and then to be able 

 to give some account, not only of the general external, but, if possi- 

 ble, of the internal characters of the different bodies. 



It should be stated that, so far as the nodules on the roots of the 

 bean are concerned, a full technical description, both of their external 

 characters and internal structure, has been given by Professor 

 Marshall Ward (' Phil. Trans.,' B, 1887, vol. 178, p. 539, et seq.). 



Reference to the descriptions which we have already given will 

 show, that the external appearance, and distribution, of the nodules 

 was very different on the roots of the peas, the vetches, and the 

 lupins. In the case of the peas there were many of what may be 

 called agglomerations of nodules, and comparatively few single ones 

 distributed on the root-fibres. On the roots of the vetches, there 



