134 BACTERIA IN THE SOIL 



upon combined nitrogen supplied within the soil, but that the 

 Leguminosge 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 cir- 

 cumstances, grew luxuriantly, and possessed on its rootlets abundant 

 nodules. The experiments of Sir John Lawes and Sir Henry Gilbert 

 at Kothamsted* demonstrated further that under the influence of 

 suitable microbe-seeding of the soil in which Leguminosse 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 (Elceagnus, etc.) when 

 these grow root nodules containing bacteria, but to all appearances 

 bacteria differing morphologically from the Bacillus radicicola 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 f that the alternative explana- 

 tions of the fixation of free nitrogen in the growth of Leguminosse 

 seem to be: 



" 1. That under the conditions of symbiosis the plant is enabled 

 to fix the free nitrogen of the atmosphere by its leaves ; 



"2. That the nodule organisms become distributed within the 

 soil and there fix free nitrogen, the resulting nitrogenous compounds 

 becoming valuable as a source of nitrogen to the roots of the higher 

 plant ; 



"3. That free nitrogen is fixed in the course of the development 

 of the organisms within the nodules, and that the resulting nitrogenous 

 compounds are absorbed and utilised by the host. " Certainly," he 

 adds, "the balance of evidence at present at command is much in 

 favour of the third mode of explanation." If this is finally proved 

 to be the case, it will furnish another excellent example of the power 

 existing in bacteria of assimilating an elementary substance. 



Experiments at Eothamsted have confirmed those of others, in 

 showing that, by adding to a sterilised sandy-soil growing leguminous 



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

 p. 129. 



t Ibid., p. 140. 



