ROOT-NODULES OF LEGUMINOSAE 



of the bacteroids, together with a few intact healthy rods that remain in the 

 soil and serve as seed material for next year's nodules. 



Root-nodules have been found in all the sub-orders of the Leguminosae 

 (Papilionaceae, Mimoseac, Caesalpineae), and they always contain bacteria 

 and bacteroids. Root-nodules without bacteria do not exist, and since Legu- 

 minosae destitute of nodules behave with regard to nitrogen just as other 

 plants do, it seems evident that the bacteria arc the nitrogen collectors. As 

 an unproved hypothesis this idea was floating in the air for a long time, until 

 the classical researches of Hcllriegel and Wilfahrt (59) placed the matter 

 beyond the reach of controversy. Their experiments proved that it was 

 possible by planting sterilized seeds in sterilized earth to grow leguminous 

 plants for some months without root-nodules, if they were well protected from 

 subsequent infection. They showed further that such plants had lost the 

 power of storing nitrogen, but regained it if the earth they grew in were inocu- 

 lated with an infusion of soil whereon Leguminosae had previously flourished. 

 Finally their researches brought to light the marked contrast between 

 nitrogen-storers like Leguminosae, and nitrogen-consumers such as wheat or 

 oats. The following table gives a few of the chief results of their investigations. 



