By Dr. MANLY MILES.
THE progress recently made in tracing the interdependent relations of living organisms is clearing up 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 agriculture.
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 leguminous 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 supplies 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-