796 ECOLOGY 



stages, Nitrosomonas, for example, oxidizing ammonia into nitrites, and 

 Nitrobacter oxidizing the latter into nitrates. The nitrates formed by 

 this process are utilized readily as a source of nitrtigen by most plants. 



Green plants on the one hand and nitrogen-fixing and nitrifying 

 bacteria on the other have a reciprocal relation of vast significance, for 

 the former produce carbohydrates and the latter nitrates, each of which 

 is of great importance for the other, as well as for all living organisms. 

 Thus there is a sort of universal symbiosis between the carbohydrate- 

 forming and the nitrate-forming organisms. The origin of this sym- 

 biosis is unknown, but it is possible that the first plants were able to fix 

 nitrogen as well as to manufacture carbohydrates, and that divergent 

 evolution has since taken place. Even among plants now living there 

 are some bacteria, notably Nitrosomonas and Nitrobacter, which can 

 manufacture carbohydrates as well as nitrites or nitrates. Sometimes 

 nitrate-forming and carbohydrate-forming organisms are in somewhat 

 close symbiosis, as in the case of Azotobacter and Oscillatoria, or Azoto- 

 bacter and Nostoc, which are two of many similar associations, where 

 both symbionts grow with unusual vigor; similarly, in salt water, Azoto- 

 bacter lives in luxuriance in the mucilage which coats the fronds of 

 Laminaria. In all of these cases the amount of nitrogen fixation is 

 greatly beyond the usual, whence it has been urged that algae can fix 

 nitrogen, though it seems more likely that such symbiosis stimulates 

 the bacteria to larger activity because of the carbohydrates which the 

 algae manufacture. An interesting but poorly understood case of sym- 

 biosis is that which exists between bacteria and myxomycetes, two 

 groups of organisms that often are closely associated; it is claimed, 

 even, that the spores of some myxomycetes, for example, Dictyostelium 

 mucoroides, fail to germinate except in the presence of bacteria, and 

 that the food of myxomycetes consists largely of such microorganisms. 



The rdle of bacteria in legume tubercles. It has been known for cen 

 turies that leguminous plants enrich the land when their roots are left 

 in the soil; furthermore, the high nitrogen content of the root tubercles 

 was known long before the tubercle bacteria or their power of nitrogen 

 fixation was discovered. Hence it is not strange that once the tubercles 

 were regarded as organs which manufacture or accumulate protein. 

 Even before it was empirically proven that Bacillus radicicola fixes free 

 atmospheric nitrogen, a comparable conclusion was reached by elimina- 

 tion, for it was shown that nitrogenous compounds do not develop in 

 sterilized soils, and that the legumes, unlike other plants, thrive in soils 



