162 



devoid of nitrogen, but it is doubtful whether they do this with- 

 out the aid of other lower organisms. In fact, it is stated in 

 one of the most recent papers on the subject by some German 

 scientists that experiments made with certain specified algae 

 prove that these plants do not fix nitrogen except by the aid of 

 certain bacteria, and that this is probably true in all cases. In 

 this connection it may be stated that Berthelot claims to have 

 proved that certain bacteria can fix nitrogen, and so enrich the 

 soil in nitrogen compounds. 



2. This leads to the second view that nitrogen is fixed in the 

 soil by means of bacteria, bacteroids, &c., and is by them con- 

 verted in the soil into some kind of nitrogenous matter, which is 

 then absorbed by green plants in the ordinary way. 



3. A third view is, to quote Professor Ward's own words,, 

 that ''the fixation of the atmospheric nitrogen can be conceived 

 of as a powerful act of the machinery of the leguminous plant, 

 urged to the necessary expenditure of energy by the stimulating 

 action of the organism in its roots." An opinion as to the 

 correctness of this view might perhaps be arrived at easily 

 if we could determine where the nitrogen is absorbed ; that is, 

 whether by the leaves and stems or by the roots. If by the 

 leaves and stems, then the hypothesis just quoted would seem 

 to be a very probable one. This is a matter extremely difii- 

 cult to deal with experimentally. Professor Ward points out, 

 however, that microscopic examination of the nodules on the 

 roots of leguminous plants indicates, by v^arious signs, that 

 they are the seat of intense physiological activity, and that 

 therefore in all probability they are the seat of the fixation of 

 nitrogen. 



4. The fourth and last theory is. that the root organisms act 

 merely as accumulators of nitrogenous material which has been 

 fixed in the soil by their means, and that the leguminous 

 plant benefits by " devouring" the bacteroids eventually, and 

 profiting by their stores of nitrogenous material. In this case 

 the bacteroids may be really more parasitic than is represented 

 by the third view of their functions ; that is, they may use 

 the root to supply themselves with nutriment, and then by 

 their vital processes to store up nitrogen. 



It it evident that a vast amount of careful and difficult 

 experimental work is necessary to decide this question, a ques- 

 tion which, however, to use Professor Ward's words, " pro- 

 mises to be of more importance to agriculture in the future 

 than any legislation as to prices, &c., that we can conceive ; 

 for if it turns out that the acquisition of free nitrogen by the 

 land, or what is the same thing, the plants growing on it, can 



