192 Scientific Proceedings^ Royal Dublin Society. 



portions of the stalks of all three of these plants, while one of them bore 

 an affected tuber. 



The second exjjeriment was carried out in pots. Eight large pots were 

 filled with contaminated bog-soil, to which a quantity of bruised and cut 

 affected tubers was further added. Two of these pots, with their con- 

 tained soil, were sterilized by suitable treatment in the autoclave. In 

 addition, two further pots were filled with non-contaminated peaty soil 

 alone. A healthy tuber was planted in each of the ten pots. In the 

 pots containing non-contaminated soil, and in those containing conta- 

 minated but subsequently sterilized soil, four plants grew whicli remained 

 healthy; and on microscopic examination, they showed no signs of the 

 presence of P. erythroseptica. In the six pots of non-sterilized contaminated 

 soil, however, only two plants grew. One of these two plants was found to 

 be quite healthy, but the other one, on microscopic examination, showed 

 the presence of P. erythrosei}tica, bearing its sexual organs, in the under- 

 ground portion of its stalk as well as in one of the rhizomes. Eighteen 

 tubers were produced on this plant, seventeen of which were healthy, 

 while one was almost entirely rotted away, leaving practically nothing 

 but its skin. This tuber was attached to tlie above-mentioned rhizome ; and 

 lining the inside of its skin the sexual organs of the fungus were found in 

 abundance. Up to the present this is tlie only tuber which has ever been 

 found in which the resting spores of the fungus have been discovered ; but it 

 is the only one which has ever been examined after the rot has been allowed 

 to complete its course, wliile the tuber remained undisturbed in the soil. 

 Further trials are being made to ascertain whetlier, if the internal portions 

 of affected tubers be allowed to rot away completely underground, tlie inner 

 surface of the skin is, as a general rule, the seat of the production of 

 oospores. 



These two experiments therefore show — 



(a) that contaminated soil may give rise to diseased plants ; 



{b) that parts of the plant other than the tubers may become attacked, 

 and that the resting spores of the fungus may be formed in them; 



(c) that the resting spores of the fungus may be found also on the 

 inside of the skin of affected tubers which complete the rotting 

 process under ground. 



From the facts given in {h) and (f) it is easy to understand how the soil 

 becomes contaminated. 



In other plots of ground at Clifden, which had been cropped with 

 potatoes for five successive years, and whicli therefore had had every chance, 

 of becoming seriously contaminated, attacked plants were plentiful in 1913; 



