109 



considerable growth at the expense of the very slight amount of ni- 

 trogenous substances normally present in water, unpurified sugar, 

 etc. 



In the next year a publication appeared in which once more 

 attention is given to the question of a possible nitrogen fixation by 

 the bacterial cultures ). It is to be regretted that the title of the paper 

 "Over ophooping van atmospherische stikstof in culturen van Bacillus 

 radicicola" - which on translation reads: "On the accumulation of 

 atmospheric nitrogen in cultures of Bacillus radicicola" - has led to 

 confusion in so far that it has often been interpreted to imply that at 

 that time BEIJERINCK claimed to have demonstrated the power of the 

 organism to fix free nitrogen. As a matter of fact, BEIJERINCK 

 maintained a very careful attitude towards the results of his ex- 

 periments, which indeed showed a certain gain of nitrogen in the 

 cultures. BEIJERINCK, however, stressed the possibility that this may 

 have been due to the presence of small amounts of nitrogenous com- 

 pounds in the air of the laboratory. Experiments undertaken to 

 settle this point were deemed to be inconclusive. 



Although the question of nitrogen fixation in pure culture of the 

 root nodule bacteria has since been a matter of much controversy, it 

 may be remarked that BEIJERINCK'S critical attitude has afterwards 

 been fully justified by the outcome of various recent investigations 

 on the subject 2 ). 



The next contribution of BEIJERINCK to the root nodule problem 

 was a short study on the nature of the so-called infection threads 

 often found in the nodules 3 ). Experimental proof is given that a close 

 correlation exists between the production of slime in pure cultures of 

 the different strains and the occurrence of the typical infection 

 threads in the corresponding host plants. The conclusion is reached 

 that these threads consist mainly of bacterial mucus, i.e., the slimy 

 cell-walls from which the bacteria themselves have been pressed out 

 more or less completely. 



For BEIJERINCK'S views regarding the way in which the legumi- 

 nous plants benefit by the infection with the bacteria the reader is 

 referred to the survey given in Part II of this book (Cf. Chapter 

 XVIII). 



b. Free oxygen in its relation to the vital phenomena of fermentation 

 organisms. 



It is self-evident that BEIJERINCK'S work in the yeast factory led 



1) Versl. en Aleded. Kon. Akad. v. Wetensch., Afd. Natuurk., Amsterdam 3de 

 Reeks, 8, 462, 1891. This paper has not been included into the earlier volumes of the 

 "Verzamelde Geschriften" ; cf., however, volume 6, 61. 



2) Cf. : E. W. HOPKINS, Soil Science 28, 433, 1929; F. E. ALLISON, Journ. Agric. 

 Research 39, 893, 1929; M. P. LOHNIS, Soil Science 29, 37, 1930. 



J) Centralbl. f. Bakt. u. Parasitenk. 15, 728, 1894. 



