106 



Prof. W. B. Bottomlev. 



The scum is most readily obtained by placing about 10 grm. of soil in a 

 nitrifying solution and incubating it for two days before adding the 

 auximone. The first trace of scum appears in 24 hours after this addition, 

 and increases as incubation is continued, until at the end of four to six days 

 it becomes so thick that it sinks to the bottom of the flask, and no second 

 scum is formed. Sub-cultures from this growth are used for test purposes. 

 It is found, however, that sixccessive sub-culturing from the original scum 

 rapidly produces an alteration in the nature of the growth, very little scum 

 being formed, and the liquid becoming turbid and bright yellow in colour. 

 Hence the necessity for obtaining a fresh scum from the prepared soil for 

 each new set of experiments. 



In order to test whether these organisms are able to indicate the relative 

 quantity of plant auximone present, six series of three flasks each were 

 arranged as follows : 



Series. Nos. 



A 1-3 Contained 100 c.c. normal culture solution. 



B 4-6 „ „ „ +4"2 parts per million silver 



All were inoculated with the scum -forming organisms, and incubated at 

 26° C. After 36 hours, series A showed no trace of scum, B an extremely 

 thick one, C a thick one, D a moderate growth, E a fair growth, but not 

 nearly so good as D, and F no appreciable growth ; hence the rate of growth 

 and thickness of scum show a progressive increase with the quantity of 

 auximone present above a certain minimum, which in this case was the 

 extract from 0'2 grm. of bacterised peat. 



The fact that this minimum amount, which represents only one part of 

 the dry silver fraction in sixteen millions of culture solution, gives a 

 formation of scum indicates the sensitiveness of the organisms to a very 

 minute trace of auximone. 



As there was a possibility of other substances present in the phospho- 

 tungstic and silver fractions being concerned in the scum formation, an 

 investigation was made of the effect of the presence of certain organic 

 substances in the nitrifying solution. The usual standard employed in tests 

 with bacterised peat extract has been the phosphotungstic fraction from 

 1 grm. of bacterised peat per 100 c.c. of culture solution, which represents 

 a solution of seventeen parts of dry substance per million of liquid.' 



fraction from bacterised 

 peat. 



C 7-9 



D 1Q-12 



E 13-15 



F 16-18 



5) 



+ 2-1 

 + 0-35 

 + 0-07 

 + 0-007 



3) 



