ASSIMILATION OF NITROGEN BY PLANTS 235 



cases, Bacterium radicicola is not present, but other similar 

 microorganisms are found. The physiological peculiarities that 

 prevent the nitrogen-fixing bacteria from living on the roots of 

 other plants are not understood as yet. Evidently there is in 

 this case the same specific adaptability of organisms toward 

 each other that is observed in most cases of real parasitism. 



54. The Nitrogen Cycle in Nature. Artificial Nitrogenous 

 Fertilizers. — The abihty of nodule bacteria in leguminous plants 

 to fix molecular nitrogen of the atmosphere is of enormous impor- 

 tance in the economy of nature in general and in the economy 

 of agriculture in particular. Leguminous plants are widely 

 distributed over the earth's surface. They are quite common 

 in every plant association. The nitrogen that they accumulate 

 is of such an amount that it is not only sufficient for their own 

 nourishment and for abundant storage in their seeds, which are 

 especially rich in nitrogen, but it also produces a general increase 

 in the nitrogen content of the soil. This enrichment of the soil 

 is due to rotting of fallen leaves, of nodules remaining in the soil, 

 and of other disintegrating parts of the plant." 



The capacity of leguminous plants to accumulate nitrogen 

 in the soil explains their popularity in agricultural practice. 

 The favorable effect of their cultivation on the subsequent yields 

 of cereals was known long before Hellriegel's time. One of the 

 classical experiments carried out by Laws and Gilbert at one of 

 the first agricultural experiment stations in Europe, at Rotham- 

 stead, England, has shown that when wheat alone was grown 

 for 10 successive years on one of two identical fields and 

 wheat in succession with leguminous plants on the other, 5 yields 

 of wheat from the second plot were equal to 10 from the first. 

 In addition, 5 harvests of leguminous plants had been gathered 

 from the second plot, and each of these contained more nitrogen 

 than did the 5 crops of wheat. Thus, the effect produced by 

 leguminous plants seems to be almost miraculous. Not only are 

 the plants themselves well nourished, but in addition the most 

 important nutritive element is left in the soil in greater abundance 

 than it was contained before the experiment. 



In general, plant production may be considered as a practice 

 in which highly valuable food and technical products are created 

 from raw material, like carbon dioxide, water, and radiant energy, 

 which cost practically nothing. The requisite amount of 



