Pethybkidge — On the Rotting of Potato Tubers. 549 



become very dark iu colour ; and the sexual organs not infrequently also 

 become similarly affected when developed in such media. 



On sterilized blocks of tissue, cut hova. potato tillers, tlie fungus grows well, 

 producing a luxuriant crop of snow-white aerial mycelium, but the growtli is 

 purely vegetative, no sexual organs or couidia being produced. 



On sterilized blocks of carrot it grows luxuriantly, forming a dense, toughish 

 felt of mycelium, covering the surface, and sending hyphae into the interior ; 

 but no sexual organs or conidia are formed. On carrot extract gelatine, in 

 which the acidity of the gelatine is neutralized previous to sterilization, the 

 growth is exceptionally good. No conidia are produced, but sexual organs 

 are fairly abundant. 



On sterilized bread (a kind of brown bread known as " Hovis " was used) 

 growth is at first slow ; but after about two weeks the entire surface becomes 

 covered with white mycelium, which also thoroughly permeates the interior 

 and fills many of the cavities. Sexual organs are abundant on the superficial 

 mycelium, but less so in the interior. No conidia are formed. 



On salep-agar the growth is comparatively poor ; the mycelial branches 

 appear somewhat stunted, and irregularities in their contours are frequent. 

 The production of sexual organs is scanty, and no conidia are formed. 



On the cotyledons of Lima Beans (which had lost the power of germination), 

 which were first allowed to swell in water, but were not sterilized, as well as 

 upon cooked Lima Bean seed-pulp, the fungus grew very well, but did not 

 develop sexual organs or conidia. 



On media derived from oats such as Quaker-Oat agard.ndi Oat-extract agar, 

 the methods of preparation of which are given in another paper (23), this 

 fungus grows exceedingly well, and produces its sexual organs in abundance. 

 The latter differ from those of Phytophtlwra infestans, which are also formed 

 on the first-named of these media in not being coloured brown, and whereas 

 conidia are produced in abundance on both of these media by P. infestans, 

 none are produced on them by P. enjthroseptica. Quaker-Oat agar, owing to 

 its opaqueness, is not a very useful medium ; but Oat-extract agar proved itself 

 to be an excellent one for use in cover-glass film cultures ; and it was largely 

 by the aid of such cultures that the mode of development of the sexual 

 organs was made out. It has already been stated that iu some of these 

 cultures conidia were met with ; but although rarely one or more appeared to 

 be in or on this medium, they were more frequently found in droplets of 

 water outside the medium proper. A single couidium was also -observed to 

 develop, but not to maturity, on one occasion, wheu a small piece of mycelium 

 from a cultiu'e on Oat agar was placed in a hanging drop of water. 



It was thought likely that couidia might develop more freely if the oat- 



