Bacterial Test for Plant Food Accessories (Auximones). 105 



The constant formation of this scum whenever the auximone was added 

 to the crude nitrifying culture from soil suggested the possibility that either 

 the scum-forming organisms were introduced with the auximone, or the 

 scum formation might be used as a specific test for auximones. To test this 

 a sub-culture was made from an original soil culture and incubated for 

 four days. No scum formed. This was then divided into two portions, 

 one of which was autoclaved at 140° C. for half an hour, after which the 

 phosphotungstic fraction from 1 grm. of bacterised peat was added to each, 

 and both were re-incubated for three days at 26° C. A thick scum formed 

 on the unsterilised liquid, but no trace appeared on the sterilised one, thus 

 showing that the scum organisms are present in the soil culture, and the 

 formation of the scum is due to the presence of the auximone. 



An examination of the scum shows that it consists of two predominant 

 kinds of organisms : a thin beaded-rod form and a spindle-shaped form. 

 The nature of the scum depends upon the relative proportion of these two 

 organisms, being crinkled and gelatinous when the beaded forms predominate, 

 and smooth and brittle when the spindle forms are in a majority. By 

 continuous plating out pure colonies of each of these forms were obtained, 

 but when grown separately in nitrifying solution plus auximone the 

 characteristic scum never appeared. Further investigations are in progress 

 as to the identity and nature of these organisms. 



The soil from which the scum-forming organisms were first obtained was 

 a rich garden soil from Kew, and an examination of other soils was made 

 in order to determine whether the organisms are of fairly constant 

 occurrence or whether they are restricted to certain localities. Upwards 

 of a dozen samples of soils, including loams, clays and gravel, from various 

 places were tested, and all were found to yield the characteristic growth in 

 the nitrifying solution with the addition of auximone. The rapidity of 

 formation of the scum, however, varied considerably, the best and most 

 rapid growth being obtained from a sample of new loam from a virgin 

 (uncultivated) field , a very good growth from some old potting mixture ; 

 and a very slow and poor growth from soil from a bed of leguminous plants. 



Although the organisms are thus found to be widely distributed and easily 

 obtainable, it became necessary for experimental purposes to obtain a 

 uniform stock from which a good growth could be readily obtained. Some 

 soil was therefore sterilised, put aside for a week, and then saturated with 

 a suspension of the scum-forming organisms. It was allowed to dry down 

 at room temperature under sterile conditions, and stored in a bottle. This 

 stock can be depended upon to yield a good growth of scum in from two to 

 three days in the presence of auximones. 



vol. lxxxix. — b. i 



