536 Scientific Proceedings^ Royal Dublin Society. 



very pronounced, but yet is characteristic. Instead of the cheesy or fishy 

 smell commonly noticeable in rotting potatoes, there is a faint pungent odour 

 reminding one somewhat of very dilute formalin or of chlorine gas. 



The internal appearances presented when an affected tuber is cut open 

 vary according to the extent to which the rot has progressed. On examining 

 the cut surface of a partially rotted tuber there is usually some contrast 

 between the healthy and diseased tissues, the latter being more wet-looking 

 and of a dirty whitish colour. This contrast is considerably greater in tubers 

 which are nearing tlie end of an attack, blackening of the diseased tissues, 

 particularly near the skin, being observable. Immediately after cutting there 

 may be no very definite line of demarcation between the healthy and diseased 

 tissue, but at times the transition is more abrupt and is marked by a blackish 

 band, which, as just remarked, is usually most pronounced towards the 

 periphery of the tuber. Fig. 2, Plate XLII, shows an affected tuber at this 

 stage, and fig. 6, Plate XLIII, is reproduced from a photograph of the two 

 halves of another tuber in the same condition. 



On exposure to air a most characteristic series of changes takes place in the 

 appearance of the diseased tissue. After a few minutes this begins to appear 

 faintly pinkish in colour, and within half an hour, at the outside, as a rule, 

 the whole surface of the diseased portion becomes tinted a deep salmon-pink 

 colour. This stage is illustrated in fig. 3, Plate XLII, and fig. 7, Plate XLIII, 

 and its appearance is so constant and characteristic that both the trivial name 

 for the disease and the specific name of the fungus causing it have been based 

 on it. 



In digging potatoes, any tubers affected with this disease, if cut or 

 wounded with tlie spade or other implement, quickly become pink on the 

 exposed surface ; hence this form of rot is easily distinguished in the field, 

 and in any doubtful case, where it is not already wounded, the tuber can be 

 cut with a knife to see whether it turns pink or not. 



On standing for a further period (some hours) still exposed to the air, the 

 cut surface of the affected portion of a tuber gradually darkens, and finally 

 becomes dark purplish brown, or even nearly black. Fig. 4, Plate XLII, 

 and fig. 8, Plate XLIII, illustrate this stage. 



No attempt has yet been made to study the exact causes of these colour- 

 changes, but they are probably of an enzymatic nature. Experiments 

 showed that they do not occur in the absence of air. For, when affected 

 tubers were cut under water and the halves allowed to remain there for 

 several hours, the pink colour scarcely developed at all, nor did the cut 

 surfaces become blackened. A faint trace of pink coloration was, however, 

 noticeab-e in the water, and when the cut halves were at length removed and 



