I*ETHYBRiDGE — On the Rotting of Potato tubers. 539 



tap, using a brush, and then soaked for some hours in a weak solution of 

 either mercuric chloride or formalin. They were then removed and allowed 

 to dry, sometimes being rinsed in alcoliol previously to drying. With a knife 

 which had been standing in strong alcohol, and from which the adhering 

 spirit was allowed to burn off immediately before use, the tubers were halved, 

 the cut being started at the still healthy end of the tuber, and small portions 

 of the affected tissues were removed by means of sterile instruments from a 

 region near the line of junction of the healthy and diseased tissues, and were 

 placed on the surfaces of layers of wort gelatine, which had previously been 

 allowed to set in Petri dishes. Out of a number of such plates, prepared at 

 one and the same time, the majority usually gave rise to pure growths of the 

 fungus. It was found necessary to avoid removing the piece of inoculating 

 tissue from anywhere in the neighbourhood of the skin, as such pieces almost 

 invariably proved to be sterile, doubtless because during the preliminary 

 steeping of the tuber in a disinfectant, some of the poison diffused through 

 the skin into the underlying tissues. 



It is quite possible also to obtain the fungus pure without having recourse 

 to the preliminary steeping, but the percentage of successes is considerably 

 lower than when the tubers are first disinfected externally. The most frequent 

 contaminations are bacteria, but since the fungus spreads in and over the 

 medium mentioned much more rapidly in the course of a few days than the 

 bacteria do, it is quite possible to obtain the fungus apart from the latter by 

 removing a small portion of its peripheral growth to a suitable culture- 

 medium after this lapse of time. 



The wort gelatine used for purposes of isolation was prepared by adding 

 10 to 12 per cent, of gelatine to ordinary beer wort prepared from malted 

 barley. This medium was slightly acid to litmus, and on it the fungus has 

 never been found to produce any reproductive organs, but it grows in a 

 purely vegetative manner only. It was, however, found later on that if the 

 medium was rendered neutral before sterilization, the fungus produced its 

 sexual organs on it. 



When portions of the fungus thus isolated were placed upon blocks of living 

 potato, prepared with aseptic precautions, or when portions were placed in 

 wounds made at the heel-ends of sound and externally disinfected tubers, the 

 characteristic form of rot was produced without any difficulty, while 

 controls treated similarly, but not inoculated, remained sound. 



Further, from such artificially inoculated tubers the same fungus has been 

 re-isolated in pure culture by the methods already described. This has been 

 done a number of times, both before the reproductive organs had been found 

 and studied, as well as subsequently. It seems scarcely necessary to enter 



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