264 SCIENCE PROGRESS. 



They grew plants from seed, leguminous and others, in a 

 sterilised glass apparatus, which may be described as a large 

 test-tube fitted with supply and exit pipes, through which a 

 known atmosphere could be circulated by means of a pump. 



The whole system was closed, but samples of the atmo- 

 sphere could be withdrawn and analysed at any time. The 

 apparatus was a beautiful and ingenious contrivance, and 

 provided for the supply of water and carbon-dioxide to the 

 foliage exposed to the light, as well as for a circulation of 

 air known to consist of oxygen and nitrogen in the free 

 state only, all nitrogen compounds being taken out. 



The principle of the experiment was this : Not only was 

 the nitrogen in the seed and soil and in the atmosphere in 

 which the plants were growing known at the beginning of 

 the experiment, but it was also known in the crop, soil and 

 atmosphere at the end of the experiment. Consequently any 

 " fixation " of nitrogen by the plant appeared as a balance 

 at the conclusion of the experiment. It would take too long 

 to describe the details, but Laurent and Schloesing proved 

 that not only did the nitrogen increase in the crop of 

 leguminous plants, but a corresponding amount had disap- 

 peared from the atmosphere surrounding them. This result 

 was only got provided these plants were infected with the 

 bacteroids, however, and well provided with nodules, as 

 described above. If no nodules appeared — the bacteroid 

 organism not being added to the sterilised soil — the legu- 

 minosae starved, as did all other plants such as cruciferae, 

 gramineae, potato, beet, etc., tried. 



At first sight this seems conclusive, but in reality it does 

 not settle the real point at issue. It does prove — and more 

 conclusively than the numerous other experiments which 

 have now placed this matter beyond all doubt — that the 

 nitrogen is taken from the air and must be taken as free 

 nitrogen, but it does not settle the question whether this free 

 nitrogen is forced into combination in the soil or in the 

 leguminous plant itself, and, if the latter, we have still to 

 ask, is the fixation due to the bacteroids, or to the symbiotic 

 compound plant (leguminous plant plus its bacteroids) ; and 

 whereabouts in the infected plant does it take place ? 



