24 Scientific Proceedings, Royal Dublin Society. 



are decidedly not in the majority of cases, as should be the ease, situated at 

 the "heel" end, where the rhizome enlarges to form the tuber? When the 

 mycelium leaves the old set, and passes into the main stalk, and thence 

 proceeds on its way to the new tuber (as it is supposed to do), why is it, 

 as previously stated, that such stalk tlius invaded at its base by a virulent 

 parasite shows no signs of this in the drooping and other unusual behaviour 

 of its foliage, as occurs in other similar cases such as Black Stalk E,ot, &c. ? 

 The truth is that it is just as difficult to reconcile well-observed facts concern- 

 ing tlie attack of the tubers witli this theory as it is in tlie case of the stalks 

 and foliage. 



In tlie plot-experiment described above, the crop was raised on August 

 22nd, about five weeks after Phytophtliora was first observed on tlie foliage. 

 During tliis time the blight liad not spread to any very great extent over 

 the plot owing to the fact that the plants were sprayed three times, 

 and when dug, they possessed but little diseased, and still plenty of 

 healthy, green foliage. As was to be expected from the nature of tlie 

 plants and the poverty of the soil, the yield was not a large one. All 

 the tubers, however, were very carefully examined, and not a single one 

 showed any traces of being attacked by Phytophthora. Being raised before 

 the foliage was badly blighted, so that a copious fall of " spores " could not 

 take place, it was to be expected that tlie tubers could scarcely have become 

 infected from tliat source ; but if they could become infected from the old 

 sets direct, surely the period available for this to have occurred can scarcely 

 be described as having been too short ? For, if the hypothetical dormant 

 mycelium had in this time been able to awake and reach the leaflets, surely 

 it could have traversed the much shorter distance to the new tubers, and yet 

 they were found to be healthy in every case ! 



It is further maintained that by means of this dormant mycelium 

 theory the outbreaks of severe and simultaneous epidemics are more 

 satisfactorily accounted for than by supposing that the destruction is due 

 primarily to the dispersion of " spores." It is imagined that infection by 

 this latter method would take place too slowly to permit of, say, the 

 destruction of whole fields within a short period, as sometimes occurs. 

 It is stated that " when a potato-plant infected with the spores of 

 Phytophthora is placed under a bell-jar in a very damp atmosphere, subdued 

 lio-ht, and high temperature — conditions most favourable to the development 

 of the parasite — it is only after a period of four or five days that the fungus 

 produces fruit on the leaves, and then only at the point of infection. On 

 the other hand, the fact is too well known that a field of potatoes — or all the 

 fields in a certaiu district — which at a given time appeared to be healthy 



