ALINIT 185 



The growth on potato very closely resembles that 

 produced by Bs. vulgatus. 



The organism stains irregularly with fuchsin. 



4. Frank, Hellriegel, and others observed that unsteril- 

 ized soils which become covered with minute green algae 

 frequently increase very considerably in nitrogen content, 

 but when protected from the light, or when the surface 

 is covered with sand or other material to prevent the 

 growth of these small plants, such soils do not become 

 richer in combined nitrogen. Schloesing and Laurent 

 found that free nitrogen of the air was absorbed and fixed 

 when the latter was enclosed in a vessel containing soil 

 covered with a coat of green algae and exposed to light. 

 The accumulation of nitrogen compounds produced under 

 these conditions may amount to .8 gr. for I kg. of soil, 

 and occurs chiefly in the thin layer near the surface. 

 Such observations led to the belief that minute green 

 algae belonging to Cyanophyceae and Chlorophyceae are 

 able to utilize free atmospheric nitrogen, but Kruger and 

 Schneidewind and Kossowitch,by careful experiments with 

 pure cultures of Cystococcus, Stichococcus and Chlorella in 

 media from which combined nitrogen is eliminated have 

 proved that this view is incorrect. No growth occurs 

 when free nitrogen is supplied to such cultures, but there 

 is luxuriant development when nitrates are provided. 

 Bouilhac obtained similar results with cultures of Nostoc 

 punctiforme. It has been found, however, that when soil 

 bacteria are added to pure cultures of these algae nitrogen 

 fixation goes on extensively. Between the algae and the 

 bacteria there is no doubt a kind of symbiosis ; the latter 

 organisms fix the nitrogen, deriving the organic com- 

 pounds essential for the process from products of carbon 

 assimilation carried on in the light by the green algae, 



