164 BACTERIA 



nitrogen, some of them at any rate could acquire nitrogen 

 brought into combination under the influence of bacteria. 

 Hellriegel found that the gramineous, polygonaceous, cruci- 

 ferous, and other orders depended upon combined nitrogen 

 supplied within the soil, but that the Leguminoscz did not 

 depend entirely upon such supplies. 



It was observed that in a series of pots of peas to which 

 no nitrogen was added most of the plants were apparently 

 limited in their growth by the amount of nitrogen locked 

 up in the seed. Here and there, however, a plant, under 

 apparently the same circumstances, grew luxuriantly and 

 possessed on its rootlets abundant nodules. The experi- 

 ments of Sir John Lawes and Sir Henry Gilbert at Rotham- 

 sted ' demonstrated further that under the influence of 

 suitable microbe-seeding of the soil in which Leguminosce 

 were planted there is nodule formation on the roots, and 

 coincidentally increased growth and gain of nitrogen beyond 

 that supplied either in the soil or in the seed as combined 

 nitrogen. Presumably this is due to the fixation, in some 

 way, of free nitrogen. Nobbe proved the gain of nitrogen 

 by non-leguminous plants (Eloeagnus, etc.) when these grow 

 root nodules containing bacteria, but to all appearances 

 bacteria differing morphologically from the Bacillus radi- 

 cicola of the leguminous plants. 



These facts being established, the question naturally 

 arises, How is the fixation of nitrogen to be explained, and 

 by what species of bacteria is it performed ? In the first 

 place, these matters are simplified by the fact that there is 

 very little fixation indeed by bacteria in the soil apart from 

 symbiosis with higher plants. Hence we have to deal 

 mainly with the work of bacteria in the higher plant. Sir 

 Henry Gilbert concludes 2 that the alternative explanations 



J Sir Henry Gilbert, F.R.S., The Lawes Agricultural Trust Lectures, 1893, 

 p. 129. 



2 Ibid., p. 140. 



