370 ANNUAL REPORT OF THE Off. Doc. 



But an aunouncement so radical and startling as this could not 

 rest on the authority of one man alone, and accordingly, llellriegel's 

 experiments, with modifications, were repeated by numerous investi- 

 gators, notably, Nobbe, Hiltner and Schmid,^* with the same results. 



In Hellriegel's experiments his plants were grown in the open air, 

 and since atmospheric air contains combined nitrogen in the form 

 of ammonia and nitrous acid, it was a question, though the soil 

 might be free from nitrogen, whether the nitrogen compounds present 

 in. the air might not be the source of nitrogen to the plant instead of 

 free nitrogen. 



Accordingly, in 1892, Atwater and Woods,^° in this country, grew 

 peas in glass cases the air of which was free from every trace of com- 

 bined nitrogen. The soil and nutrient solution used to water and 

 fertilize the plants were also nitrogen free. At the end of the ex- 

 periment, covering 85 days, 27 plants had made an average growth 

 of 29 inches, with a total gain of 242 mg. of nitrogen. It was fur- 

 thermore noted, that as a rule the largest gains were in those plants 

 which showed the largest tubercle development. Thus, the authors 

 conclude, "The free nitrogen of the air was alone available to the 

 plants, and the gain must have been by the acquisition of free ni- 

 trogen." 



There is, therefore, no longer any doubt remaining in the minds of 

 scientists that leguminous plants possess the power of utilizing the 

 free nitrogen of the atmosphere. 



It has also been shown that they do this in proportion to the 

 poverty of the soil in available nitrogen. In other w^ords, the plant 

 does not utilize the function of appropriating free nitrogen unless 

 forced to do so. 



A legume will thrive in the presence of an abundance of avail- 

 able nitrogen in the soil, or will respond to a liberal application of 

 nitrate of soda. And it does so because it is easier for it to utilize 

 a form of nitrogenous plant food already prepared for it than to go 

 through the more elaborate process of appropriating the free nitro- 

 gen of the air. 



But if the soil be deficient in nitrogenous food the legumes will 

 extend their appropriating energies along these new^ lines, eventually 

 overcome their soil environment and come off the victor. 



On this principle is based the value of leguminous plants as soil 

 enrichers. 



It has been stated, and has perhaps been fairly w^ell established, 

 that the increase in nitrogen in legumes is proportionate to the 

 development of tubercles. 



In his experiment with beans, Halsted^^ found that in every case 

 there was a decided increase in yield of beans on soils where several 

 successive crops of beans had been grown, over the yield where 



