PROGRESS IN AGRICULTURAL SCIENCE. 491 



PROGRESS IN AGRICULTURAL SCIENCE. 



By Dr. MANLY MILES. 



THE progress recently made in tracing the interdependent rela- 

 tions of living organisms is clearing np some of the obscure 

 problems in the nutrition of plants that have a direct bearing on 

 the processes of evolution and the applications of science in agri- 

 culture. 



Since the discovery of the composition of the atmosphere, the 

 problem of the sources of the nitrogen of vegetation has given 

 rise to a wider range of experimental investigation and discussion 

 than any other in vegetable physiology. The evidence appeared 

 to be conclusive as to its source in certain families, including the 

 cereals, while the larger supplies of nitrogen obtained by legu- 

 minous plants were not fully accounted for. 



The experiments of Boussingault, in France, and the elaborate 

 investigations at Rothamsted, in England, seemed to show that 

 atmospheric nitrogen is not appropriated, to any extent, by the 

 leaves of plants, and that the soil is the main or sole source of the 

 nitrogen of vegetation. 



Wheat and barley were the leading cereals under experiment, 

 as field crops, at Rothamsted ; and it was found that, while they 

 contained less nitrogen in their composition than leguminous 

 crops, they were specifically benefited by nitrogenous manures. 

 On the other hand, leguminous crops, which obtained larger sup- 

 plies of nitrogen from the soil, were not benefited by nitrogenous 

 manures, and they grew luxuriantly on soils that did not furnish 

 the cereals with their comparatively limited supplies of nitrogen. 



These apparently paradoxical results are now explained, in 

 part at least, by investigations made within the past five years by 

 Hellriegel and Willfarth, Ward, Prazmouski, and others, which 

 have been fully verified by experiments at Rothamsted which are 

 still in progress. Former experiments showed that leguminous 

 plants obtained nitrogen from some source, or under conditions 

 that were not available for the nutrition of the cereals, and it was 

 evidently not obtained from the atmosphere. 



It was suggested that the tubercles observed on the roots of 

 leguminous plants had a direct relation to the appropriation of 

 nitrogen ; but most observers looked upon them as abnormal and 

 of no physiological significance. 



The latest investigations, however, show, beyond the shadow 

 of a doubt, that these " tubercles " or " nodules " are the results of 

 infection by microbes, and that "the relation between the roots 

 and the bacterial organisms is a true symbiotic one, each develop- 



