1892.] on Micro-organisms in their Belation to Chemical Change. 527 



our atmosphere can be assimilated or utilised as food by plants. 

 This question was answered in the negative by Boussingault about 

 fifty years since ; the problem was again attacked by Lawes, Gilbert, 

 and Pugh about thirty years ago, and their answer was also in the 

 negative. In the course, however, of their continuous experiments 

 outcrops, Lawes and Gilbert have frequently pointed out that whilst 

 the nitrogen in most crops can be accounted for by the combined 

 nitrogen supplied to the land in the form of manures and in rain 

 water, yet in particular leguminous crops, such as peas, beans, vetches, 

 and the like, tbere is an excess of nitrogen which cannot be accounted 

 for as being derived from these obvious sources. The origin of this 

 excess of nitrogen in tbese particular crops they admitted coukl not 

 be explained by any of the orthodox canons of the vegetable physio- 

 logy of the time. The whole question of the fixation of atmospheric 

 nitrogen by plants was again raised in 1876 by a very radical 

 philosopher, in the person of M. Berthelot, whilst the most conclusive 

 experiments were made on this subject by two German investigators, 

 Prof. Hellriegel and Dr. Wilfarth, who have not only shown that this 

 excess of nifrogen in leguminous crops is obtained from the atmo- 

 sphere, bnt also that this assimilation of free nitrogen is dependent 

 upon the presence of certain bacteria flourishing in and around the 

 roots of these plants, for when these same plants are cultivated m 

 sterile soil the fixation of atmospheric nitrogen does not take place. 

 Moreover, the presence of these microbes in the soil occasions the 

 formation of peculiar swellings or tuberosities on the roots of these 

 plants, and these tuberosities, which are not formed in sterile soil, are 

 found to be remarkably rich in nitrogen, and swarming with bacteria. 

 [Lantern-slide of nodules on roots of sainfoin (Lawes and Gilbert).] 

 Extremely important and instructive in this respect are the experi- 

 ments of Prof. Nobbe, who has not only confirmed the results 

 mentioned, but has endeavoured to investigate the particular bacteria 

 which bring about these important changes, and he has indeed suc- 

 ceeded in showing that in many cases each particular leguminous plant 

 is provided with its particular micro-organism which leads to its 

 fixation of free nitrogen. Thus he found that if pure cultivations of 

 the bacteria obtained from a pea-tubercle were applied to a pea plant 

 there was a more abundant fixation of atmospheric nitrogen by this 

 pea-plant than if it was supplied with pure cultures of the microbes 

 from the tubercles of a lupin or a robinia; whilst similarly the robinia 

 was more beneficially affected by the application of pure cultures from 

 robinia-tubercles than by those from either pea-tubercles or lupin- 

 tubercles. [Lantern-slides exhibiting Nobbe's experiments on pea 



and robinia.] 



This subject of the source of nitrogen in leguminous plants has 

 again been taken up by Sir John Lawes and Dr. Gilbert at Eothamsted, 

 and their recent results fully confirm the observations of these foreign 

 investigators that it is partially derived from the free atmospheric 

 nitrogen through the agency of bacteria in the soil. 



