60 



Frank does not think, however, that the tubercle organism is itself 

 the agent of the assimihitiou of nitrogen. This power belongs to the 

 jilant and the presence of the microbe simply- stimulates the plant to 

 greater growth in poor soils, and hence indirectly increases its power of 

 assimilating nitrogen. In his experiments he found that the whole 

 activity of the plant was stimulated by tlie i>resence of the microbe, its 

 growth including its development of chlorophyl, and its assimilation of 

 carbon as well as of nitrogen, lie urges tbat other phints besides 

 legumes have the power of nitrogen assimilation, though i>i'rlia[)s in 

 less degree. Even some algje, he thinks, can do this. He claims further 

 that his experiments show that the legumes can assimilate atmospheric 

 nitrogen even when they are free from tubercles. This they can not do 

 readily in i)Oor soil, since their general vigor is here too snuiU, but 

 in rich soils they do assimilate great quantities of nitrogen and he 

 thinks that they get it from the air. Moreover the cultivation of the 

 tubercle organism in artificial culture does not iiulicate that it can 

 nourish itself on atmospheric nitrogen. For these reasons he concludes 

 that there is no ground for attributing the nitrogen assimilation solely 

 to the microbe, but that it is really a property of the plant, stinudatcd 

 to excess in certain cases by the presence of the microbe. WJiilc then 

 the tubercle organism is of value for certain species of legumes growing 

 in poor soil, it is of no value in rich soils, ami for certain species it is of 

 no value at all. He found no reason lor believing that dilVercnt species 

 of tubercle organisms were associated with ditlerent species of legumes, 

 and concluded thiit in ordinary soils in which legumes had been grow- 

 ing the microbe was present in suilicient quantities to])roduce an abun- 

 dance of tubercles. 



Prazmowski* has reached somewhat dillcrent conclusions. His 

 experiments were fewer in number, but were performed with greater 

 precautions against accidental errors. Tiie plants were all grown in 

 specially i)repared vessels, so arranged that the contents could not come 

 in contact with any air that had not first been filtered through cotton to 

 deprive it of germs. The stem of the plant i>rotruded through a 

 specially guarded oi)ening in the toj) of tlie vessel, and the leaves were 

 thus exposed to the air. In the globe-shaped culture vessel he placed 

 sand which had been most thorimghly purified of all traces of nitrogen- 

 ous matter by washing, and freed of all microbes by sterilizing at a high 

 heat. All water used in the culture was conducted to the bottom of the 

 vessel by a sterilized glass tube, so arranged as to prevent the possi- 

 bility of contamination by bacteria from the air. Into this vessel of 

 purified saiul ho introduced solutions containing the plant foods with 

 which he wished to experiment, and thoroughly sterilized the whole 

 apparatus by heat. He then introduced a. germinating seedling, using all 

 antiseptic precautions. During the experiment he caused a stream of 

 filtered air to4)ass through this soil, and all water used was thon)ughly 



* Landw. Vers. Stat., 38 (1890), pp. 5-62; Experiiuent Station Record, vol. ii, p. 686. 



