fixation of fkee nitrogen. 137 



Evidence as to Fixation of Free Nitrogen. 



It can hardly be said that there remains an unsolved prob- 

 lem in the matter of the sources of the nitrogen of our 

 non-leguminous crops — of wheat, of barley, and of grasses, as 

 representatives of the great Natural Order of the Graminese ; 

 of turnips, representing the Cruciferse ; of some varieties of 

 beet, representing the Chenopodiacese ; and of potatoes of 

 the Solanese. It must be admitted to be quite otherwise so 

 far as our leguminous crops are concerned. 



It is nearly a century since the question whether plants Early Re- 

 took up, or evolved, free nitrogen became a matter of experi- f*dicaUn 

 ment and of discussion; and it is more than half a century that plants 

 since Boussingault commenced experiments to determine ^otdraw 

 whether plants assimilate free nitrogen. fromthe 



From his results he concluded that they did not ; and air - 

 those obtained at Eothamsted more than thirty years ago 

 confirmed the conclusions of Boussingault. In fact, we con- 

 cluded that under the conditions of those experiments, which 

 were those of sterilisation and enclosure, in which, therefore, 

 the action both of electricity and of microbes was excluded, 

 the results were conclusive against the supposition that, under 

 such conditions, the higher chlorophyllous plants can directly 

 fix free nitrogen, either by their leaves or otherwise. 



It may, in fact, be concluded that, at any rate in the case 

 of our gramineous, our cruciferous, our chenopodiaceous, and 

 our solaneous crops, free nitrogen is not the source. Never- 

 theless, we have long admitted that existing evidence was 

 insufficient to explain the source of the whole of the nitrogen 

 of the Leguminosse ; that there was, in fact, a missing link ! A missing 



Limiting the discussion here mainly to the question of the lmk ' 

 sources of the nitrogen of the Leguminosee, it is generally 

 admitted that all the evidence that has been acquired on lines 

 of inquiry until recently followed, has failed to solve the 

 problem. During the last few years, however, the discussion 

 has assumed a somewhat different aspect. 



The question still is, whether free nitrogen is an important The new 

 source of the nitrogen of vegetation generally, but especially doctnne ' 

 of the Leguminosse ? But whilst few now assume that the 

 higher chlorophyllous plants directly assimilate free nitrogen, 

 it is nevertheless supposed to be brought under contribution 

 in various ways ; but especially by being brought into com- 

 bination under the influence of micro-organisms, or of other 

 low forms, either within the soil itself, or in symbiotic growth 

 with a higher plant. 



Professor Atwater made numerous experiments, both on fj™^ s 

 the germination and on the growth of peas. In eleven out of ments, 



