312 EFFECT OF CHEMICAL AGENTS [Cn. XI 



culty may be partly met by sterilizing the soil, but this prob- 

 ably produces also other changes than the death of the bacteria. 



The most careful observations of the last five years have not 

 supported FRANK'S generalization. Here and there there 

 have been observers who, like LIEBSCHER ('92) and STOKLASA 

 ('96), believe they have evidence for the direct assimilation of 

 free nitrogen by the cells of phanerogams. But the evidence 

 for the contrary opinion is predominant. 



The experiments which speak for the theory that green 

 plants cannot directly make use of free nitrogen are not in 

 unison. Thus, some indicate that in non-leguminous as well 

 as leguminous plants nitrogen of the air is indirectly made use 

 of through the action of the bacteria of the soil, while accord- 

 ing to others it would seem not to be made use of at all. To 

 the first class belong the experiments of PETERMANN ('91, '92, 

 and '93) with barley, of NOBBE and HILTNER ('95) with 

 mustard, oats, and buckwheat, and of PFEIFFER and FRANKE 

 ('96) with mustard. To the second class belong the experi- 

 ments of SCHLOSING and LAURENT ('92 and '92 a ) with 

 various plants, DAY ('94) with barley, and AEBY ('96) with 

 mustard. In the second class, however, the experimental con- 

 ditions did not favor the development of the bacteria of the 

 soil. The experiments of PETERMANN were, however, carried 

 out upon a very large scale and under practically normal con- 

 ditions and showed a marked difference between the acquisi- 

 tion of nitrogen by barley growing in an unsterilized soil and 

 in a sterilized one. Likewise NOBBE and HILTNER, and 

 PFEIFFER and FRANKE, were careful to rear plants under 

 normal conditions, so that their results are worthy of especial 

 consideration. They agree that there is an acquisition of 

 nitrogen by the plant growing in normal soil and that this 

 occurs only when the soil is unsterilized. We conclude then 

 that probably phanerogams, like algse, can use the free nitro- 

 gen of the air as food only after it has been converted into 

 nitrates by the action of the nitrifying organisms the bac- 

 teria of the soil. 



Turning now to animals, whose nutrition is often compared 

 with that of fungi, we find an absence of knowledge on the 

 subject of the nutritiveness of free nitrogen. It is clear that 



