24 JOURNAIv OF THE 



lieved by some that the equilibrium between free and 

 available nitrog-en is in part preserved in this way. 

 When letjfuminous plants are g-rown under normal con- 

 ditions there are formed upon the roots small nodular 

 or wart like protuberances varyino- in size from that of 

 a pin head to a pea. They are o-enerally called tuber- 

 cles. Microbes are found associated with all tubercles, 

 and are plainl}^ the cause of their formation. Leg-u- 

 minous plants, when g-rown in sterilized soil, have no 

 tubercles and require nitrog'enous food for their growth. 

 If soil infusions are added to the cultures in sterilized 

 soil, tubercles are formed and the plants thrive with- 

 out the addition of nitrogenous manures. It is then 

 clearly not an inherent power of the plant, but depends 

 •upon the presence of the tubercles, which are caused 

 by the soil organisms. Atwater and Woods (Conn. 

 Station Ann. Report, 1889), found in a series of eight3^ 

 nine experiments that, in all cases, where there was 

 tubercular formations there was also an increased g-ain 

 of nitrog'en, being' the gfreatest where there was the 

 g'reatest number of tubercles. So far as our present 

 knowledg"e extends root tubercles are confined to the 

 Leg'uminosai. They are readily produced on the roots 

 of any member of this family, either by inoculation, 

 the addition of small portions of crushed tubercles, or 

 soil infusions containing- the organisms. The concur- 

 rent opinion of all the investig*ators of this subject is 

 that the formation of tubercles is caused by the soil 

 microbes, and upon their formation depends the power 

 of the plants to assimilate free nitrog-en of the air. 

 Here the agreement ceases. There are about as many 

 different opinions in regard to the way in which this 

 nitrogen accumulation takes place as there have been 

 investig"ators of the subject. However it may be, it is 



