47 



E. G. Mulder: Copper and Molybdenum 



shown. In agreement with the results obtained by Arnon and 

 Stout (2) a very poor growth of tomato plants was obtained in 

 the absence of molybdenum (Plate 7). In order to show the 

 effect of molybdenum on nitrate reduction in barley and tomato, 

 these plants were grown in nutrient solutions without molyb- 

 denum and with a moderate supply of nitrogen. When the 

 plants had grown for some weeks in this medium slight symptoms 

 of molybdenum deficiency, as well as of nitrogen deficiency, were 

 observed. Abundant potassium nitrate was then added to the 

 cultures and in addition some of them were given sodium molyb- 

 date. Some days later the plants were harvested and nitrate, 

 as well as organic nitrogen fractions, were determined in leaves, 



Tablb 7 : Effect of molybdenum on denitrification (^incubation time 10 days) : — 



• loitial nitrate concentration! 13. i mg N per bottle (50 cc). 



stems and roots. In the absence of molybdenum practically no 

 nitrate reduction had taken place (Table 8). 



The nitrate content in leaves and stems appeared to be high, 

 whereas the organic nitrogen fractions did practically not increase. 

 In the plants with molybdenum added an intensive nitrate reduc- 

 tion and formation of organic nitrogen compounds had taken 

 place. With barley similar results were obtained. Apparently 

 molybdenum has the function of a catalyst in nitrate reduction 

 in microorganisms as well as in higher plants. 



The Importance of Molybdenum in the Fixation of Gaseous 



Nitrogen:— BoRTELS (3) in 1930 was the first to show that molyb- 

 denum is indispensable for the nitrogen fixation of the free-living 

 nitrogen-fixing bacterium Azotohacter chroococcum. The ques- 



