Chap, in The absorption of Nitrogen 343 



crops the amount of nitrogen in the produce varied 

 according to the amount of nitrate supplied, whether they 

 were watered with the soil extract or not. With the 

 Leguminosae things went differently, those which were 

 allowed to grow in the sterilized soil behaved like the 

 cereals, the nitrogen they contained varied with the nitrate 

 supplied ; but those in whose culture the soil extract had 

 played a part showed a remarkable gain. They con- 

 cluded that ' the Papilionaceae are distinguished from the 

 Gramineae in not being dependent on the soil for their nitro- 

 genous food. The sources of nitrogen which the atmosphere 

 affords have for these plants the highest importance. They 

 alone can suffice to bring them to a normal or full develop- 

 ment.' 



In a subsequent series of cultures, they found that the 

 growth was as great when the air supplied was freed from 

 nitric acid and ammonia as when no such precaution was 

 taken. Hence they concluded the source to be the free 

 atmospheric nitrogen. 



Hellriegel and Wilfarth found by careful observation of 

 their cultures that the apparent power of absorbing nitrogen 

 from the air was associated with the presence of peculiar 

 nodular outgrowths on the roots of the plants concerned, 

 and that there was some correspondence between the 

 amount of the absorption and the luxuriance of the nodules 

 or tubercles. 



These nodular outgrowths appear to have been noticed 

 by the older writers ; they are alluded to by Malpighi 

 (1687), de Candolle (1825), and Treviranus (1853). They 

 were first examined by Woronin in 1866, as were certain 

 others resembling them, growing on the roots of the alder. 

 He described them as containing fungoid organisms, differ- 

 ing, however, in the two cases, the lupins containing 

 bacteria, the alder a fungus apparently of the genus 

 Sckinzia. The cells in the interior of the tubercles were 



