J. Henderson Smith 40 



For example, we saw (Tables V, VI) that as we raised the amount 

 of phenol from 0-4 per cent, to 0-7 per cent, the survivor curve tended 

 more and more to assume the logarithmic shape. On reducing the 

 temperature we were able to use 1-0 per cent, phenol, and yet the re- 

 sulting curve was frankly sigmoid, and even with 1-25 per cent, the 

 sigmoid character is still present. This is intelligible enough, if we con- 

 sider that the relationship between resistance and kilhng time will be 

 greatly modified by the lowering of the temperature; but it is difficult 

 to understand if we suppose that the eflect of the low temperature is 

 merely to slow the reaction. 



It seems clear that we must take into account th(; change of i-ate in 

 different grades produced by the change of piienol strength; and it is 

 a considerable simplification if, when this is allowed for, both the sigmoid 

 and logarithmic curves can be derived from the normal or nearly normal 

 distribution of resistances in the spores of the suspension. 



I have pleasure in thanking Mr R. A. Fisher for assistance and 

 criticism in the more mathematical aspects of this work. 



Summary. 



1. It is shown that if Botrytis spores be exposed to the action of 

 0-4 per cent, phenol, the spores do not all die simultaneously, but some 

 die in a few minutes and some not till two or three hours have elapsed. 

 The curve showing the numbers surviving at different times has a sig- 

 moid shape. 



2. If the strength of phenol be progressively raised, the curve be- 

 comes less and less sigmoid, approaching the logarithmic type of curve. 



3. With the same suspension it is possible to obtain either a log- 

 arithmic or a sigmoid curve according to the strength of phenol used. 



4. Both types of curve are shown to be exphcable on the assumption 

 that the individual spores differ in resistance and that a frequency curve 

 showing the distribution in the resistance grades approaches the normal 

 curve. 



5. The influence of the number of- spores used is shown to be very 

 considerable; and the consecutive transition from the sigmoid to the 

 logarithmic type occurs, whether we raise the phenol strength, keeping 

 the spore number constant, or reduce the spore number keeping the 

 phenol constant, or use younger and younger spores. 



Ann. Biol, vin 



