116 Sir J. B. Lawes and Prof. J. H. Gilbert. [Jan. 9, 



Though the symbiosis is useful to both, the plant gains most, for 

 it is the more powerful, and sooner or later overcomes the bacteria, 

 to the multiplication of which it sets limits and finally absorbs the 

 substance of the latter. Being the stronger, the plant directs the 

 symbiosis." 



If we understand the foregoing statements rightly, it is assumed 

 that the bacteria acquire their nutriment, including their nitrogen, 

 from the protoplasmic cell-contents of the higher plant ; and that, on 

 the other hand, the contents of the bacteroid cells are resorbed. "Tn 

 other words, the plant utilises the substance of the bacteria." But it 

 is obvious that, so far as the nitrogen of the bacteria is derived from 

 the plant itself, the latter is not a gainer in a quantitative sense. 



It is further assumed, that the activity of the process depends — " on 

 the quantity of nitrogenous compounds at the disposal of the roots. 

 In a soil rich in nitrogen the tubercles go on developing unhindered, 

 become large and typical, .... in poorer soils they attain no 

 great size, &c." Here, then, combined nitrogen in the soil is sup- 

 posed to be the so'urce of the nitrogen of the bacteria, and that they 

 develop the more, the greater the supply of it. Undoubtedly, however, 

 the nodules may develop very plentifully in a nitrogen-free soil, and 

 there may be great gain of nitrogen, if only the soil be suitablj' 

 infected. Indeed, the tendency of the evidence so far at command 

 seems to show, that both the development of the nodules, and the gain 

 of nitrogen, may be the greater in the poorer, but properly infected 

 soil. Further, so far as the combined nitrogen of the soil is the 

 source of the nitrogen there is no gain of it. 



Marshall Ward says, however, that no decision is arrived at as to 

 whether the nitrogen is got from nitrogen compounds or from the 

 free nitrogen of the air, nor as to what advantage accrues to the 

 bacteria and the host-plant respectively. But he adds that the 

 symbiosis is advantageous to both the plant and the bacteria ; the 

 latter feeding on the sap and the cell- contents, whilst the plant obtains 

 nitrogen by means of the bacteria. 



It is obvious, however, that if the nitrogen of the bacteria is 

 derived from the plant itself, it will be quantitatively no gainer by 

 resorbing it. Nor would there be any such actual gain of nitrogen 

 as there undoubtedly is, if the source of the nitrogen, either of the 

 parasite or of the host, were essentially the supplies of combined 

 nitrogen within the soil. 



The most probable alternatives seem to be — 1. That, somehow or 

 other, the plant itself is enabled, under the conditions of the symbiotic 

 life, to fix the free nitrogen of the atmosphere by its leaves ; a sup- 

 position in favour of which there seems no evidence whatever. 2. That 

 the parasite utilises and fixes the free nitrogen, and that the nitroge- 

 nous compounds f jrmed are taken up by the host. On such a supposi- 



