n6 THE SPIRIT OF THE SOIL 



scum organisms made their appearance. They did 

 not appear when the accessory food bodies were 

 derived from dry maize or dry peas. It seems evi- 

 dent that these scum-forming organisms are able to 

 serve as a satisfactory test both for auximones and 

 for the accessory food bodies of Hopkins. 



Tests were also undertaken with fresh and rotted 

 manures. The result with these was that it was 

 found that the quantity of auximones present 

 increased with the progressive decomposition of the 

 organic matter of manure. The amount of it was 

 relatively small. In two-year-old rotted manure, 

 compared as against bacterized peat, a better scum 

 was obtained with the fraction from ^ gramme of 

 peat than with 10 grammes of the manure. 



A source from which the auximones can readily 

 be derived is the leguminous plant. Thus the 

 Phosphotungstic acid fraction derived from 

 jV gramme of bean nodules gave a very thin film of 

 scum. The fraction from \ gramme of bean root 

 gave a fair growth, while with T V gramme a good 

 growth was obtained. When, however, beans were 

 grown in sterilized sand without the formation of 

 nodules no scum at all appeared, even though the 

 extract of T V gramme of root was used. 



As regards the scum-forming organisms used for 

 the test, it may be noted that they require no organic 

 Carbon, but, like the nitrifying organisms and the 

 sulphur and iron bacteria, they are able to assimilate 

 Carbon dioxide by chemo-synthesis. They are 

 unable to live on nitrates, but must derive their 

 Nitrogen from an Ammonium salt. 



