18 Proceedings of the Royal Irish Academy. 



season, were carried on in an incubator at 25°-27° C. in order to obtain the 

 results more quickly. 



All of what we may call our early, critical inoculation experiments were 

 carried out as thus described ; later on it was found that the mercuric chloride 

 treatment could safely be dispensed with, and the inoculations were not in 

 every case made exactly through the cut-surface of the rhizome. In all such 

 cases, it is scarcely necessary to add, control-tubers stabbed, but not inoculated, 

 were used as before ; and throughout the whole of the work such control- 

 tubers remained, without exception, sterile and unaffected, and up to about 

 the end of September, as will be mentioned later on, we had not a single case 

 of failure to rot when the organism, presently to be described in detail, was 

 inoculated into a tuber in the manner indicated. 



The difference between the tubers inoculated with the organism and the 

 controls was clearly evident, as a rule, after a period of twenty-four hours. By 

 that time the open end of the stab in the case of the controls showed a very 

 slight discolouration, if any, and had practically dried up. In the inoculated 

 tubers, on the other hand, the wounds were moist-looking, and almost black 

 in appearance. This difference increased as time went on, very frequently a 

 small quantity of a dark-looking liquid being exuded from the stabs in the 

 inoculated tubers, while the skin in the immediate neighbourhood of the stab 

 became blackened. The differences were even more strongly marked when, 

 after a total period of seventy-two hours, the tubers were cut through 

 longitudinally. In the case of the controls there was an empty cavity where 

 the stab had been made, the walls of which were quite firm and free from any 

 marked discolouration. In the inoculated tubers this cavity was filled with a 

 soft, discoloured, pulpy mass, and the adjacent tissues were becoming similarly 

 soft and pulpy, and were in some cases blackened. Fig. 5, Plate III, shows 

 an inoculated tuber cut longitudinally through the stab. 



Further details regarding the action of the organism on the tubers will be 

 found when its cultural characteristics are being discussed. From some of the 

 tubers, in which the characteristic rot had been thus artificially produced, 

 platings were made of the decayed tissues, with the result that the organism 

 used for inoculation purposes was re-isolated in pure culture. 



In addition to isolating it from diseased potato-stalks, we were successful 

 in obtaining the same organism twice out of three times from affected 

 tubers borne on deceased plants. The failure in the third instance was in all 

 probability due to the fact that the rot in the tuber had, at the time of the 

 experiment, reached an advanced stage, when not only the pathogenic organism 

 but also an abundance of other forms were present. In this case the tuber pre- 

 sented a large cavity when cut open, lined by a fairly thick layer of a blackish 



