198 LECTURE X. 



nacce, Cycadece, Conijerce, etc. The assumption has also been made that 

 the hypomycetes, which often exist symbiotically with the roots of higher 

 plants, have a similar function to that of the nodules. These experiments 

 have not yet been completed. Many observations on the wild plants 

 seem to indicate a wider distribution of such symbioses. We know of 

 many plants which, year by year, always grow in the same spot with the 

 usual profusion, while many others suddenly spring up and after a short 

 " period of blossoming " gradually fade away. In this way the dominant 

 species in a meadow, and especially in a dump-heap, may follow one another 

 in rapid succession, probably because these short-lived plants rely exclu- 

 sively on the nitrates of the soil. 



It is of great importance to us in considering this subject to realize 

 that nature possesses ways and means of assimilating the free nitrogen 

 of the air. On the one hand, nitrogen is set free; and on the other, it is 

 recombined. We are not prepared to state the relations existing between 

 these two processes, whether they maintain an equilibrium, or whether 

 the liberation of nitrogen far exceeds that of recombination. It would 

 be very interesting to know the compounds into which these organisms 

 convert the nitrogen. 1 At present we have no knowledge concerning this. 

 We assume that the final substance produced is albumin, which is then, in 

 part, assimilated by the plants with the help of fermentation. 



The discovery that ordinary nitrogen can be directly assimilated closes 

 the chain of the nitrogen cycle, which had apparently been broken open by 

 the discovery of the denitrifying organisms. We have forgotten to men- 

 tion one point. We shall see later, when considering the inorganic nutrient 

 materials, that the earth possesses the power of " fixing " certain constitu- 

 ents. This applies, for instance, to the important elements, phosphorus, 

 potassium, ammonia, etc., which are so necessary for the develop- 

 ment of the plants. As soon as their solutions come in contact with cer- 

 tain constituents of the earth they are changed into insoluble compounds, 

 and are thus protected from being washed away by rain-water. The salts 

 of nitric acid, the nitrates, behave quite differently. They are not absorbed 

 by the earth. They are readily soluble in water, and are continually being 

 washed away, carried to brooks and streams, and finally appear in the ocean. 

 The amount of nitrogen abstracted yearly from the soil in this manner 

 is really enormous. K. Brandt 2 estimates it at 40,000,000 kilograms 

 per year. We are confronted with the question of how this large amount 

 of nitrogen is returned to the general nitrogen cycle. That this must take 



1 Recently it has been found possible to convert technically large amounts of atmos- 

 pheric nitrogen into its compounds. 



2 Wissenschaftlichen Untersuchungen. Report of the Kommission zur Untersuchung 

 der deutschen Meere, 1899 and 1901. Cf. also, E. Schulze: Schweizer. landwirtschaftl. 

 Zentralblatt. 1902. 



