CHEMICAL CHANGES 223 



nodules when the bacteria assume the bacteriodal form. That the 

 plant receives nitrogen from the tubercles is shown by analyses 

 by Stoklosa 1 at different stages of growth : 



Nitrogen in 

 tubercles 

 Lupines Per cent. 



Flowering stage 5.22 



Fruit forming 2.61 



Plant mature 1.73 



Fig. 51. Branched bacteria from a clover nodule. 



The percentage of nitrogen in the tubercles becomes less as 

 they grow older. 



Effect of Conditions on Tubercle Formation. The conditions 

 here discussed are: (i) nature of the bacteria; (2) effect of 

 fertility of the soil; (3) effect of salts. 



Nature of Bacteria. The more active the bacteria, the less 

 quickly do they assume the bacteroidal form; the stronger the 

 plant, the more easily it causes this change. A strong plant may 

 indeed prevent the entrance of the bacteria into its roots and con- 

 sequently no nodules will be formed. The nature of the bacteria 

 seems to be modified by the host plant. That is to say, the 

 bacteria in the nodules of one plant may not inoculate other plants 

 very well ; it may produce nodules upon them, but not of such size 

 1 Exp. Sta. Record 7, p. 922. 



