Pethybridge and Murphy — On Phrjtophthora infestans. 575 



After the lapse of periods of time varying from nine to twenty days small 

 portions of the browned tissue were removed from eight of the tubes and 

 transferred as carefully as possible to slants of wort-gelatine. This was no 

 easy matter, since, as mentioned above, the diseased tissue becomes very 

 tough, and to cut off small portions from the affected cylinders situated in the 

 bottoms of test-tubes without at the same time running considerable risk of 

 contamination presents some difficulties. In two cases Penicillium, in another 

 an unidentified mould, and in a fourth bacteria developed on the wort- 

 gelatine slants. We liave every reason for believing that these contamina- 

 tions arose during the process of transference, for in the remaining four cases 

 the wort-gelatine slants remained absolutely sterile. Finally, the remaining 

 portions of the eight affected cylinders were removed and subjected to 

 microscopical examination, when absolutely no bacteria and no fungus other 

 than P. infestans could be discovered. 



A somewhat similar experiment was carried out on whole tubers. These 

 were washed, disinfected, dried, and inoculated with the pure culture. After 

 about three weeks the tubers were cut open under strictly aseptic conditions, 

 when the browned, diseased areas were found as usual. Small portions of 

 this browned tissue were removed and planted on one oat-agar and three 

 wort-gelatine slants. The three latter remained absolutely sterile, while on 

 the oat agar P. infestans developed in characteristic fashion, but no other 

 organism of any kind was present. 



If therefore there is au organism which is associated with P. infestans in 

 causing the brown discolouration, it does not grow either on wort-gelatine or 

 on oat agar ; and since the pure culture originally used had been obtained 

 from a long series of periodical transferences from oat agar to oat agar, it 

 is practically impossible to believe that any such organism could have been 

 present; and we are forced to the conclusion that P. infestans is alone 

 responsible for the well-known browning. 



Jones found, as stated above, that oogouia-like bodies were formed in his 

 pure cultures on raw potato. We have never seen them on this medium ; 

 but it must be admitted that our search for them here has up to tlie present 

 not been so prolonged or so thorough as has been the case with other media. 

 A pure culture on Lima bean agar, which Professor Jones was good enough 

 to send us early in 1911, contained the fungus in active growth and in a 

 normal condition of virulence ; but microscopic examination of this particular 

 culture failed to reveal the presence of any bodies suggestive of sexual 

 organs in it. 



(2) Grouth in rate Potato-juice. — Six medium-sized tubers were well 



washed, peeled, chipped into small portions, and then well squeezed through 



4q2 



