242 Plant Physiology 



Ten years later Winogradski isolated Clostridium 

 Pasteurianum, an organism which proved to be capable 

 of fixing free nitrogen when grown in the absence of air 

 (oxygen), also similarly capacitated in the presence of air 

 when associated with other bacteria utilizing free oxygen. 

 Since that time much work has been done. Several other 

 species of soil bacteria having the power of fixation have 

 been isolated, these latter being included under the genus 

 Azotobacter. They are common and important in arable 

 soils containing a relatively small amount of combined 

 nitrogen; moreover, their activity is enhanced by the 

 presence of considerable lime and by general fertility as 

 regards the other mineral nutrients. From the majority 

 of experiments thus far reported it does not appear that 

 the addition to the soil of cultures of these organisms has 

 occasioned increased nitrogen fixation. 



135. Fungi. - The investigation of nitrogen fixation 

 by fungi has yielded many data of interest, although there 

 is some conflicting evidence. Fixation of nitrogen by 

 fungi was reported as early as 1862, but the more impor- 

 tant work has been done since 1895. Among several 

 saprophytic and parasitic species employed, Saida secured 

 maximum fixation with cultures of Phoma Betce, a fungus 

 normally parasitic upon sugar-beets. 



Several observers have reported fixation for a few of the 

 common molds of soils and decaying vegetation, including 

 Aspergillus niger and Penicillium glaucum. In all cases 

 the organisms were grown in pure cultures, either in the 

 absence of combined nitrogen, or in the presence of very 

 small quantities of such compounds. In no case does the 

 amount of nitrogen fixed amount to more than a few 

 milligrams. 



