Petrie, — On Vegetable Physiology. 435 



that exist in the atmosphere. Thus, probably, the only re- 

 maining assumption we can make is that leguminous plants 

 have the power of making use of the free nitrogen of the air. 

 To their nutrition, and especially to their assimilation of 

 nitrogen, the so-called tubercles and the organisms that 

 dwell in them stand in the closest active connection." 



Many other observers have taken part in the elucidation of 

 this subject, and Hellriegel's results have been abundantly 

 verified both in England and on the Continent. The most 

 rigorous confirmation has been supplied only two or three 

 years ago by Laurent and Schlcesing. These inquirers grew 

 leguminous plants from seed in a sterilised glass apparatus of 

 great beauty and ingenuity, which provided for the supply of 

 pure water and carbon-dioxide to the foliage exposed to the 

 light, as well as for the circulation of air known to consist of 

 oxygen and nitrogen in the free state only, all nitrogenous 

 compounds being taken out. Samples of the air could at any 

 time be taken out and analysed, and at the close of the experi- 

 ment the whole of the air affected by the growing plants could 

 be collected and examined. In this way Laurent and Schlce- 

 sing proved not only that the nitrogen increased in the grow- 

 ing plants, but that a corresponding amount of nitrogen had 

 been removed from the air that surrounded them. This result 

 was got only when the experimental plants w^ere infected with 

 bacteroids and well provided with root-nodules. If the 

 bacteroid organisms were not added no tubercles appeared, 

 and the plants starved. 



This research proves in the most conclusive way that 

 the nitrogen fixed by infected leguminous plants is taken from 

 the air, and must be taken as free nitrogen. 



So far as leguminous plants are concerned, the modern 

 results are of course in direct conflict with those which Bous- 

 singault was thought to have established about the middle of 

 the century. The discrepancy is due to the fact that bacteroid 

 infection was excluded in Boussingault's experiments, one 

 main object of which was to show thiit humus and its imme- 

 diate derivatives were not necessary, and, indeed, not directly 

 available, for the nitrogenous nutrition of plants. 



The fact of the fixation of the free nitrogen of the air by 

 infected leguminous plants being thus finally and firmly 

 established, the exact mode or process of the fixation re- 

 mained to be explained. 



At first various opinions on this subject were held by com- 

 petent inquirers. Hellriegel's and Laurent and Schloesing's 

 investigations have made some of these untenable. One view, 

 which is still maintained by Frank of Berlin, was that the 

 Leguminosge took up the free nitrogen of the air by their leaves. 

 The experiments of Hellriegel, already described, and others 



