196 LECTURE X. 



and fresh water. The following observations of Ktihn l are mentioned to 

 give an idea of the significance of the activity of these nitrifying bacteria. 

 A field, which for twenty years had not had any nitrogenous fertilizer 

 added to it, gave an average return of 1976 kilograms of grain per hectare. 

 Not only was there no decrease in the annual yield due to the gradual 

 removal of the nitrogen of the soil, but it actually showed an increase of 

 11.6 per cent in grain produced. There was annually withdrawn from a 

 hectare of land in crops of rye, from 25-30 kilograms of nitrogen. This 

 amount of nitrogen must have been taken from the air and transferred to 

 the soil. Even the fallen leaves in forests assimilate nitrogen by the activity 

 of the bacteria contained within themselves. It is not at all impossible 

 that these bacteria are the pioneers in converting decomposed rock into 

 arable land. 



It is a well-known fact that some plants, for instance the legumes, 

 enrich the soil with nitrogen, while others only deplete it. The practical 

 farmer utilizes this fact by not planting cereals year after year on the same 

 soil, but rotates the legumes and the grains. Hellriegel 2 and Willfarth 3 

 have satisfactorily explained the whole subject as a result of their experi- 

 ments. They proved that the legumes assimilated the nitrogen, and 

 showed that this formation was intimately connected with the enlarge- 

 ments of the so-called " root-nodules or tubercles " of these plants. It was 

 also shown that the legumes could be made to grow nodules on sterilized 

 soil if infusion of ordinary soil be sprinkled over it. There must evidently 

 be micro-organisms present in the soil which cause the formation of these 

 nodules. The infusion obtained from the soil loses its activity on being 

 heated. The gramince act entirely different. They are not influenced in 

 their consumption of nitrogen by any infusion from the soil. Their nitrogen 

 assimilation is dependent on the nitrates already present in the soil. Free 

 nitrogen is of no service to them. The legumes, on the other hand, are 

 entirely independent of 'any increase of nitrates. If the legumes are 

 grown in sterilized soil, they behave like the graminse. They lose the 

 faculty of fixing the free nitrogen, and rely entirely upon the nitrates in the 

 soil. The following experiments are referred to as illustrating the assim- 

 ilation of nitrogen through the nodules of the legumes. Schloesing and 

 Laurent 4 cultivated legumes in sterilized soil and in sterilized glass cyl- 

 inders. The amounts of carbon dioxide, oxygen and nitrogen in the air 



1 Fruhlings landw. Ztg. p. 2, 1901; quoted by F. Czapek: Biochemie der Pflanzen: 

 G. Fischer, p. 131, 1905. 



2 Tageblatt Naturforscher-Vers. Berlin, 1886, p. 290. 



3 Tageblatt Naturforscher-Vers. Berlin, Wiesbaden, 1887, p. 362 ; Zeit. Ver. Riiben- 

 zuckerind. Beilageheft, 1888, p. 234, and Ber. botan. Ges. 7, 138 (1889). For further 

 literature see J. Vogel: Zent. Bakt. u. Parasitenkunde 15, 11, 33, 1905. 



4 Compt. rend. Ill, 750 (1890); 113, 776(1891); 115, 881, 1017 (1892) ; Ann. Inst. 

 Pasteur. 6, 65 and 824 (1892). 



