59 



Laurent. But althougb, as will be seen below, there is a consensus of 

 all experimenters as to the general relation of the tubereh's to the 

 assimilation of nitrogen, the ditt'erences regarding details lead to almost 

 diametrically opposite views as to the interpretation of the phenomena. 

 The most imiiortant of the recent publications on this subject are those 

 of Frank and Prazmowski. These observers have been working for 

 several years and have each published several papers. In 1800 each 

 {)ublislied a long account of the results of his previous work, and witli 

 it a summary of his views on the whole <iuestion. These two pa[)ers 

 were published so nearly at the same time that neither contains any 

 reference to the other. A review of these two papers will give us the 

 best outline of the present condition of the discussion. 



The extensive investigations of Frank* were carried on with several 

 species of legumes which he cultivated in pots containing soils of vari- 

 ous kinds. In one series a nitrogen-free soil and in a second series a 

 rich humus was used. Sterilization of the soils was accomplished by 

 heating, and he admits that the heat produced various chemical changes, 

 especially in the humus, which interfered with the success of his experi- 

 ments. His results indicated that legumes do gain nitrogen- from the 

 air and that the power in some cases at least is connected with the for- 

 mation of root tubercles. According to bis experiments, however, dif- 

 ferent species of legumes differ in this respect. Beans, forexam]>le, 

 show no greater amounts of nitrogen when tubercles are present than 

 when they are absent. For such plants the tubercle is to be regarded 

 simply as a parasite. In the case of the lupine and the pea, the tuber- 

 cles are of more value. Of course none of these plants grow vigor- 

 ously in nitrogen-free soil under ordinary conditions, but when they are 

 provided Mith the tubercle organism their growth is more vigorous, 

 they develop more leaves, and they assimilate more carbon and more 

 atmospheric nitrogen. But according to Frank even these plants gain 

 no advantage from the tubercles if they are growing in rich soil. When 

 growing in soil containing nitrogen they have no need of the microbes, 

 being able to assimilate [)lenty of nitrogen without them. Plants grown 

 in the rich humus used in some of his experiments showed an equal 

 growth and fixation of nitrogen whether they developed tubercles or 

 not. In some experiments plants without tubercles actually develoi)ed 

 better than those with tubercles, a fact which he thought might be 

 explained by the injurious action of the sterilization on the chemical con- 

 dition of the soil, Frank concluded then that under ordinary condi- 

 tions the tubercles are of no use to the plant. But when the plants 

 are growing in poor soil, and especially in one lacking in nitrogen, they 

 have not of themselves sufficient vigor to develoj) and assimilate nitro- 

 gen from the air. The microbes of the tubercles under these conditions 

 serve as a stimulant to the plant. Under their inHuence the plant 

 takes on new vigor, grows normally, and assimilates nitrogen from the 

 only possibly source — the air. 



•Landw. Jahrb., IdOO, pp. 523-640. 

 3348— No. 1 5 



