266 SCIENCE PROGRESS. 



■ 



infected tubercles. But whether it occurs by means of the 

 stimulated protoplasm of the cells of the tubercle, or by 

 means of the protoplasm of the bacteroids here placed in 

 such circumstances that they can, by oxidising and destroy- 

 ing large quantities of the carbo-hydrates abundantly brought 

 to them, force nitrogen into combination on their own account 

 — as Winogradsky's results suggest that other bacteria can 

 do quite apart from any host plant — must for the time being 

 remain an open question. Meanwhile, however, another 

 aspect of this question of the fixation of free nitrogen had 

 forced itself into the foreground. 



In Laurent and Schloesing's experiments it was found 

 that although sterilised soil alone was unable to fix nitrogen, 

 sterile sand on which certain mixtures of common soil, 

 algae and bacteria were sown did fix it. Frank (28) had 

 previously stated that algae could fix nitrogen in daylight 

 but not in the dark ; and Laurent and Schloesing showed 

 that it was essential that their algae should not be covered 

 up from access of light, e.g., by a layer of sand. 



In all these cases it turns out that we have to do not 

 with the fixation of nitrogen by algae, but with the action 

 of symbiotic combinations of algce and bacteria, for Kosso- 

 witsch (29) has recently shown that the algae alone, isolated 

 in pure cultures by Beyerinck's method, are incapable of 

 fixing the nitrogen, whereas if soil-bacteria of various kinds 

 are allowed access to the algae, in the slimy cellulose walls 

 of which they grow and establish symbiotic unions, con- 

 siderable, and in some cases very large, amounts of free 

 nitrogen are fixed, and this power is increased by adding 

 to the symbionts /zas - / such carbo-hydrates, viz., sugars, as the 

 algae will provide. These experiments not only throw 

 much light on the whole question, but they possibly enable 

 us to understand Berthelot's results, in which nitrogen- 

 fixing bacteria could only do the work if supplied with 

 plenty of carbo-hydrates, and bring the whole matter into 

 relationship with Winogradsky's results. In fact, as the 

 authors referred to indicate, and ais Duclaux (30) has re- 

 cently pointed out, it looks as if the whole question of nitro- 

 gen fixation, whether by bacteria of soil, the nodule 



