THE ELEMENTARY CONSTITUENTS OF PROTOPLASM 41 



the surrounding oxygen. The mechanism by which this organism is able 

 to fix free nitrogen, and the nature of the first product of the assimilation are 

 not yet ascertained. Such an assimilation will serve to the organism as a 

 source of energy, since the application of heat is necessary for the dissociation 

 either of ammonium nitrite or of nitrous acid into nitrogen and water, as is 

 seen from the following equation ; 





HN0 2 Aq. + 308 Cal. = H + N + 0, + Aq. 

 NH 4 N0 2 Aq. + 602 Cal. = 2N + 4H + 20 + Aq. 



In addition to this spontaneous fixation of nitrogen by humus, a method 

 has long been known to farmers by which the fertility of a soil can be in- 

 creased without the application of nitrogenous manures. 

 If a plot of land is to be left fallow .it is a very usual 

 custom to sow it with some leguminous crop such as sain- 

 foin. Careful experiments by Boussingault, Lawes and 

 Gilbert, and others, have shown that the growth of almost 

 any leguminous crop in a soil poor in nitrogen may result 

 not only in the production of a crop containing much com- 

 bined nitrogen, but also in an actual increase of nitrogen in 

 the soil from which the crop is taken. It was then shown 

 by the last two observers, as well as by Schloesing and 

 Laurent, that the power of a leguminous crop to enrich the 

 soil with nitrogen was dependent on the presence on the 

 roots of certain small nodules which had been described 

 long before by Malpighi (Fig. 14). They showed also that 

 the production of these nodules took place only as a result 

 of infection. Beans grown in sterilised sand produced a 

 plant free from nodules, which however grew very scantily 

 unless nitrogenous manure were added to the sand. Such a 

 crop derived the nitrogen for its growth from the added 

 nitrogen, the total amount of which in the soil was there- 

 fore t diminished by the crop. If however the sterilised 

 sand were treated with an infusion of root nodules from 



another plant without the addition of any combined vetch with nod- 

 nitrogen at all, the beans developed nodules on their roots ules. 

 and grew luxuriantly, and at the termination of their 

 growth the soil was richer in nitrogen than at the commencement. On 

 microscopic examination the protoplasm which makes up these nodules 

 is found to be swarming with small rods (Fig. 15), and it was shown by 

 Beyerinck that these rods are bacteria and can be cultivated in media 

 apart altogether from the plant. We have thus an example of a class of bac- 

 teria which, like those of humus, are able to assimilate the free nitrogen of the 

 atmosphere, but, unlike them, can only effect this assimilation in a condition 

 of symbiosis, i.e. living in the growing tissues of a leguminous plant. Similar 

 nodules have been described on the roots of other plants which can grow in a 



