28 Proceedings of the Royal Irish Academy. 



line mentioned above may be absent. In others, especially where the stab 

 was a large one, its sides become blackened, probably owing to the access of 

 air to its interior, for, on exposure to air, the pulp above mentioned gradually 

 darkens, often becoming almost black. 



The reaction of this pulp to litmus was carefully studied. It may be 

 recalled that, in tubers naturally infected from the parent plant, the diseased 

 tissues give at first a distinctly acid reaction ; later on, after blackening has 

 occurred, the reaction becomes alkaline. Care being taken to avoid the 

 naturally acid sap of the healthy tissue, it was found that, if a little of the 

 pulp were placed on a blue litmus paper, it was immediately reddened. If, 

 however, a little of the same pulp was placed at the same time on a piece of 

 red litmus paper, a slight blue colouration was produced. Thus at this stage 

 the pulp shows an amphoteric reaction, the acidity being, however, more 

 strongly marked than the alkalinity. On exposure to air, however, the 

 reaction gradually becomes alkaline. If a moistened red litmus paper be left 

 on the inside of the lid of a dish in which potatoes are rotting, it gradually 

 becomes blue, as is the case with cultures in artificial media. The odour 

 produced by rotting tubers is but a faint one, somewhat resembling a mixture 

 of stale fish and potatoes. There is no smell of butyric acid. 



It will be convenient to give at this point a brief account of the action of 

 the organism on tubers of the same variety, of different ages, and on those of 

 different varieties. Up to the end of September the inoculations had always 

 been made on tubers freshly, or very recently, dug for the purpose. On 

 starting experunents to ascertain the susceptibility of different varieties of 

 potatoes to the attacks of the organism, tubers were at first used which, in 

 some cases, had already been dug and stored in sprouting-boxes for a week or 

 two. Whereas up to this time we had no case of failure to rot when the 

 organism was inoculated into a tuber, we now found, somewhat to our 

 surprise, that some varieties, which had previously taken infection easily, now 

 did so only with difficulty or scarcely at all. The results of the following 

 experiment seem to show it is more difficult to produce a vigorous rotting in 

 ripe and lifted potatoes than in unripe ones still in the ground and attached 

 to the plant. 



On October .3rd seven Champion tubers which were freshly dug, and seven 

 others which had been dug and boxed four weeks earlier, were inoculated 

 with what was ascertained to be a quite virulent culture of the organism. 

 With the seven fresh tubers the rot proceeded vigorously in all cases. Rotting 

 also occurred in six out of the seven old tubers ; but it was decidedly less 

 vigorous than in the fresh tubers, and in three cases it apparently came to a, 

 standstill after a few days. 



