310 EFFECT OF CHEMICAL AGENTS [Cn. XI 



These results, abundantly confirmed by MOLISCH ('95), seem 

 to show that unless bacteria are present algae can build up free 

 N into nitrogenous compounds only slowly, if at all.* 



While SCHLOSING and MUNTZ, BERTHELOT, and others were 

 gaining an explanation of the enrichment of fallow ground, 

 HELLRIEGEL ('86) and WILFARTH were making their investi- 

 gations upon the cause of the enriching action of leguminous 

 crops, which led them to the conclusion that it was due to the 

 fixation of nitrogen in plants with root-nodosities containing 

 bacteria, the compounds thus formed by the symbiotic bacteria 

 being directly assimilated by the plant. This conclusion has 

 been repeatedly sustained for leguminous plants (Fig. 86). 



The question whether green phanerogamous plants other 

 than legumes can make use of free atmospheric nitrogen is one 

 which is still in hot debate, and it is not an easy one to answer. 

 The calm conviction, based chiefly upon the excellent work of 

 BOUSSINGAULT ('60), and LAWES, GILBERT, and PUGH ('61), 

 that nitrogen is not thus obtained was rudely shaken by the 

 paper of FRANK ('93) in which that author stated that he finds 

 that nitrogen is removed from the air by non-leguminous 

 plants plants, moreover, which are not known to have 

 bacteria living in their roots. Consequently he is of opinion 

 that perhaps the fixation of free nitrogen may be carried 

 on by any living plant cell. 



The difficulties in the solution of the problem may thus be 

 set forth. It is recognized that all growing plants make use 



* One "crucial test" of FRANK still requires an explanation. If the free 

 nitrogen is "fixed" by the aid of bacteria, the process should go on in the dark. 

 Experiment shows that it does not do so. This difficulty KOSSOWITSCH over- 

 comes by the assumption that the activities of the bacteria are dependent upon 

 certain carbohydrates which the plant can afford them only in the light. 



