ON THE GROWTH OF LEGUMINOUS CROPS. 53 



nitrogen, if such took place at all in connection with vegetation. But 

 his own very distinct final conclusion against the supposition of such 

 fixation by the agency of the lower organisms, seemed to indicate the 

 necessity for caution in accepting much of the evidence which has been 

 accumulating during the last few years. 



It is evident that since experimenting with non-sterilised materials, 

 and in free air instead of in closed vessels, has become more general, 

 there has been a great accession of evidence which is held to show the 

 fixation of free nitrogen. But, not only are the gains in some cases 

 very small, and in others very large, but the modes of explanation are 

 very different. 



Thus, the various modes of explanation of the observed gains of 

 nitrogen are : that combined nitrogen has been absorbed from the air, 

 either by the soil or by the plant ; that there has been fixation of free 

 nitrogen within the soil, by the agency of porous and alkaline bodies ; 

 that there has been fixation by the plant itself ; that there has been 

 fixation within the soil (or by the plant), by the agency of electricity ; 

 and finally, that there has been fixation under the influence of lower 

 organisms, either within the soil itself, or in symbiotic growth with the 

 higher plant. 



The balance of the evidence recorded, is undoubtedly much in favour 

 of the last mentioned mode of explanation. But of all the recent 

 results bearing upon the subject, those of Hellriegel and Wilfarth, with 

 certain leguminous plants, seem to be by far the most definite and 

 significant. Accordingly, as we stated in October, 1888, in a postscript 

 to our paper " On the Present Position of the Question of the Sources 

 of the Nitrogen of Vegetation." (Phil Trans., 1889.) it had been 

 decided to institute somewhat similar experiments at Rothamsted. 

 A preliminary series was, in fact, then in progress ; and a more 

 extended one has been undertaken in the present season, 1889. The 

 results of these experiments show conclusively that, by the addition to 

 the experimental soil, of a small quantity of the watery extract of a 

 soil containing the appropriate organisms, there was greatly increased 

 growth, and considerable gain of nitrogen ; and there was, coincidently, 

 a considerable development of the so-called leguminous nodules on the 

 roots of the plants. 



The conclusion is, not that the leguminous plant had directly utilised 

 free nitrogen ; but that the gain was due to the fixation of nitrogen in 

 the growth of the lower organisms in the root-nodules ; the nitrogenous 

 compounds so produced, being taken up and utilised by the leguminous 

 plant. It would seem, therefore, that in the growth of leguminous 

 crops, such as Clover, Vetches, Peas, Beans, Lucerne, &c., at any rate 

 some of the large amount of nitrogen which they contain, and of the 

 large amount which they frequently leave as nitrogenous residue in the 

 soil for future crops, may be due to atmospheric nitrogen so derived. 

 It has yet to be ascertained, however, under what conditions a greater 

 or less proportion of the total nitrogen of the crop will be derived, on 

 the one hand from nitrogen compounds within the soil, and on the 



