680 SCIENCE PROGRESS 



observations have been extended by Nobbe, Hiltner, Frank, 

 Moore, and others ^ and showed that many strains of this 

 bacillus exist and that a favourable influence on artificial 

 infection is only noticed when cultures obtained from nearly 

 related families are used, e.g. bacteria from peas will produce 

 nodule formation in peas, but not in robinia roots. It is also 

 interesting to note that the Japanese Soja hispida very often 

 does not grow nodules in European soil, but produces them 

 when inoculated with Japanese soil. Nothing is known so far 

 about the initial products formed by the bacteria from nitrogen, 

 but it is assumed that proteins are finally produced which are 

 made use of by the plant. 



Still another stage in the nitrogen cycle must be mentioned, 

 since it helps us to understand the conditions of plant and 

 animal life in sea-water. By the solvent agency of rain-water, 

 soluble nitrogenous substances such as nitrates are drained 

 out of the soil and finally carried by rivers into the sea. An 

 enormous amount of nitrogen (Brandt calculates it at 40,000,000 

 gr. per annum) is thus lost to the dry land, and this is largely 

 in excess of that nitrogen removed from the sea in the same 

 period in the form of food fishes and other animals. The 

 question arises, how these large quantities of nitrogen are again 

 returned to circulation, and in this direction the discovery made 

 by Baur in 1902 is of the greatest importance. He demonstrated 

 the general occurrence of denitrifying bacteria in sea-water and 

 mud. These continually throw again into circulation the 

 nitrogen carried into the sea by the rivers, or which has 

 accumulated there owing to the decay of plant or fish life. 

 The discovery of the denitrifying bacteria in sea-water also 

 helps us to solve another apparent contradiction. On the 

 dry land the tropical and subtropical regions are more thickly 

 populated by plant and animal life than the regions nearer the 

 poles, whereas in the sea just the reverse is the case. In 

 accordance with this the nitrogen content of southern warmer 

 sea areas is smaller than that of northern colder ones. Now 

 it has been shown that the development of the denitrifying 

 bacteria stands in close relationship to temperature. Whilst 

 at the temperature of the tropical seas (25-30°) the nitrogen 

 evolution is most marked, the denitrifying bacteria are con- 



^ For a complete list of the literature from Pliny to 1901, see E. Jacobitz, 

 Centralb.f. Bad. II. vol. vii. 1901, 783. 



