61 



sterilize*!. In this wiiy lie reduced the possibility of the iiiHuence of the 

 contamiiiiitiii;,'' iiiicrohes to the h)\vest terms. Alter the experiment he 

 made an examination of his soils to see if he had succee<led in prevent- 

 in<^ the access of coutaminatiuj; organisms. In most of his experiments 

 this had not been aceom{)lished, but some of the soils did ap[)ear sterile, 

 even after tlie plants had been growing for weeks, and the results of 

 these experiments did not difler from the others. A special advan- 

 tage of this method of experimenting was in his knowingexactly what his 

 plants had to feed upon, tliiis avoiding all errors produced by rain or 

 dust or from unknown constituents in his soils. The great value of his 

 ex])eriments, therefore, lay in his so controlling all of the conditions that 

 he could directly determine the action of the tubercle organism. 



Piazniowski's experiments comprised four series: (1) Plants grown 

 in sterilized soil, fed with nitrogen-free food, and watered with steril- 

 ized water; (2) plants grown in sterilized soil, fed with nitrogen-free 

 food, and watered with a water which contained some of the tubtrcle 

 microbes obtained from pure cultures of these organisms; (.'5) plants 

 grown in soils su[)plied with calcium nitrate, in addition to other salts, 

 and watered with sterilized water; (4) plants grown in soils with the 

 nitrogen ration and watered with water containing the tubercle microbe. 

 The results in all his experiments were uniform. In all cases where 

 the microbe was added in the water, tubercles developed, the plants 

 were vigorous, and showed by final analysis an increase of nitrogen. 

 In the nitrogen-free soils watered with sterile water, there were 

 no tubercles and no increase in nitrogen over that contained in the 

 seed, while the inoculated i)lants in the same soils all showed an 

 increase of nitrogen, sometimes as much as sixfold. In plants fed 

 w ith a nitrogen ration there was in all cases an imnease in nitrogen, 

 but the inoculated plants always showed a greater increase than those 

 not inoculated. 



Prazmowski also carried through a series of experiments with water 

 cultures, but with less success. In water cultures containing no nitro- 

 gen he found exactly the same results as in his soil cultures. But the 

 plants which he grew in water cultures containing a nitrogen ration 

 and inoculated with the tubercle organism, did not do so well. Some 

 injurious disease atlected them (there were only two exi)eriments) and 

 prevented their normal growth. Their leaves turned yellow and they 

 did not develop tubercles iu the normal manner, although inoculated 

 with the tubercle orgainsm several times. As was to be exi)ected, when 

 analyzed these plants showed less nitrogen than the inoculated plants 

 which developed normally on the nitrogen ration. 



Prazmowski does not think it has yet been positively determined 

 whether the nitrogen fixed by these plants is derived from the free 

 nitrogen of the air or from some form of combined nitrogen, though lie 

 regards it as much more probable that it is the free nitrogen. Ilell- 

 riegel's experiments have shown that the legumes can gain nitrogen 



