Untiring research by many men and in many places has taught us that it is the mysterious force in living protoplasm that in its aggressive way reaches out and appropriates the restless molecules of atmospheric nitrogen; that though it destroys it also builds up. Practical experience had taught the ancients that crops of the legume family, crops like clover, beans, lupines, etc., do not exhaust the soil to such an extent as do crops not belonging to the same family. They had learned that after a crop of clover they could raise a larger crop of wheat. Why it was so they did not know, nor did the many generations of farmers who followed them; yet not knowing they availed themselves of the advantages that time had pointed out to them. It was reserved for the men of our generation, for men equipped with the methods of our own day, to illuminate the darkness, to unveil for us still another of nature's mysteries, to show us an intelligent way for replacing the unceasing losses of nitrogen. It was scarcely more than fifteen years ago that Hellriegel and Wilfarth published a series of wonderfully conceived and wonderfully exact experiments that decided for all time a much-debated question, which for a century had taxed the ingenuity of the foremost scientists of Europe. What Boussingault with all his mental penetration and clearness of vision had failed to accomplish, what Lawes and Gilbert with all their painstaking care and admirable equipment had failed to achieve, the German investigators had made clear. They showed conclusively that in the root nodules of leguminous plants there are found certain bacteria that in a way still unknown to us enable the host plant to make use of the gaseous atmospheric nitrogen. We do know that there is a continual struggle between the plant and the invading bacteria; we are justified in believing that the bacteria, compelled by the plant, unlock to it the hitherto inaccessible store of nitrogen. It was in this wise, partly, that the nitrogen accumulation in our soils resulted; it was in this wise that the rich prairie soils, containing at times as much as twenty thousand pounds of combined nitrogen per acre to a depth of one foot, had acquired that nitrogen. This dwelling together of two distinct forms of life with mutual benefit resulting is known as symbiosis, and the symbiotic life of leguminous plants and of the organism known as Bacillus radicicola has made possible to a great extent the terrestrial life of to-day. Yet there is another phase of the question, a phase that but a few years ago had not been recognized. There are bacteria in the soil that can avail themselves of atmospheric nitrogen without the aid of leguminous plants. Recent work in this field of research indicates that such fixation of atmospheric nitrogen is of vast significance, of greater moment, perhaps, than the fixation of nitrogen by legumes. To understand more clearly the relations existing among the many
