574 Scientific Proceedings, Royal Dublin Society. 



than over still living areas. The diseased areas on the surface of an inocu- 

 lated tuber are distinctly harder to the touch than the still healthy areas, and 

 they are also tougher when cut with a knife. As a rule, the darkening of 

 the skin precedes its sinking in. 



On cutting open such inoculated tubers the tissues show the mottled 

 brown or rusty markings so characteristic of the attacks of the fungus, as 

 seen in naturally affected tubers. The extent of the browning depends largely 

 on the time which has elapsed since inoculation took place, but periiaps also 

 to some extent upon the individuality of the tuber.' 



It is commonly supposed that this brown discolouration of the dead tissue 

 is the result of the action of the fungus in question; but Matruchot and 

 Molliard maintain that this is not the case. They state that aseptically 

 obtained cylinders cut from the tissues of tubers and artificially inoculated 

 with P. iii/estans remain for an indefinite period white and firm, retaining 

 the same aspect as non-inoculated controls, and simply drying up like the 

 latter do. We have paid considerable attention to this question of the 

 browning of affected tissue, and find that it does occur in pure cultures of the 

 fungus on cylinders of raw potato-tissue prepared aseptically ; and we cannot 

 but conclude that tlie browning is due to the action of the fungus, as is 

 generally believed. 



One of our critical experiments on this point deserves to be described in 

 detail. Twelve cylinders of living tissue were prepared under as strictly aseptic 

 conditions as possible, and were transferred with the greatest possible pre- 

 cautions to sterile test-tubes. These were then kept for a period of ten days at 

 room (summer) temperature, when close scrutiny showed that nine of them 

 were sterile, whilst the other three had become contaminated. Each of these 

 nine cylinders was then inoculated from a pure culture of P. infestans. At 

 the time of using this culture it was subjected to microscopic and also cultural 

 control,^ to make sure that it was what it purported to be — namely, a pure 

 culture. 



The nine inoculated cylinders produced a good growth of the fungus and 

 they became typically browned, as we have observed in other pure cultm-es. 



' Experiments carried out by one of us since this paper was written show that different varieties 

 of potatoes differ considerably in their reaction towards the fungus. In a variety like " Shamrock," 

 which in the field is practically immune to the blight both as regards foliage and tubers, the tubers 

 rot much more slowly when inoculated with P. infestans than do those of a variety such as " British 

 Queen," which possesses no marked resistance to the disease. 



^ Practically all of the common moulds, as well as species of Fusarium, &c., will develop on 

 wort- gelatine, as will also the majority of ordinary bacteria. F. infestans makes absolutely no 

 growth on this medium. Portions from the pure culture when placed upon wort-gelatine slants 

 produced absolutely no growth ; the slants remained absolutely sterile. The microscope revealed no 

 foreign organisms in the culture. Hence we concluded that it was pure. 



