96 BACTERIA AND THE NITROGEN CYCLE 



to assimilate nitrogen from the nitrates in the soil. The table (III) on 

 p. 91 shows this very clearly. In this semi-parasitic mode of life the 

 Leguminosae resemble the Rhinanthaceae, Thesinm, and some other green 

 plants, of which the roots graft themselves on to the roots of adjacent 

 plants of other species. These function as hosts, but it is not known what 

 substances they yield up to their parasites. 



Since it is evident that the soil of every field in which Leguminosae 

 grow, and probably indeed soil of all kinds, contains the bacteria of root- 

 nodules, attempts have naturally been made to obtain them direct from 

 the soil. Up till now these experiments have not been attended with 

 success, nor is it known whether the bacteria live and multiply in the soil, or 

 whether they exist merely in the form of spores (as yet unknown) which 

 are quickened into life by contact with the roots of leguminous plants. 



But although it has been hitherto impossible to isolate the bacteria of 

 root-nodules from soil, Winogradsky has discovered another soil bacterium 

 which fixes atmospheric nitrogen (64). It has been named Clostridium 

 Pasteuriamtm, and belongs to the group of butyric ferments. It can be 

 isolated in a nutritive solution containing sugar (as a source of carbon) and 

 mineral salts, nitrates or other nitrogen compounds being of course rigidly 

 excluded. The sugar is broken up into butyric, acetic, and carbonic acid, 

 and hydrogen, besides some by-products of uncertain composition, and at 

 the same time nitrogen is taken from the atmosphere and stored up in the 

 body of the cells. The more sugar there is present (i.e. the more energetic 

 the fermentation), the greater is the amount of nitrogen taken up, as the 

 following figures show : 



Dextrose in nutritive N. in nutritive N. in the crop 



solution. solution. after fermentation. 



Grammes. Milligrammes. 



1 o 3.0 



2 O 2-9 



3 o 8-1 

 6 o 12-8 



Possibly the nitrogen is fixed by nascent hydrogen, and ammonia 

 formed as the first product. The bacterium in pure culture is an anaerobic 

 actively motile bacillus \vhich grows in slimy masses. It is considerably 

 different in appearance from the bacteria of root-nodules, becoming spindle- 

 shaped during sporulation (hence the name Clostridium) and, like other 

 butyric bacteria, it stains blue with iodine. 



What the conditions are under which Clostridium Pasteuriamim exists 

 in nature, and whence it obtains its carbon, is unknown. Should sugar be 

 a necessity it could not grow in unmanured soil, but might occur in situa- 

 tions where the processes of fermentation and putrefaction give rise to a 

 variety of organic compounds. 



