tuTHYBKlDGii — On the Rotting of Potato Tubers. 541 



saprophytically on the various media, which will be dealt with in the next 

 section of this paper. 



(«) Mi/ceUuni. — The mycelium resembles very closely tliat of other species 

 of Phytophthora. It is much branched, devoid of transverse septa, and filled 

 witli granular protoplasm in its younger portions, but empty and freely 

 provided with cross-walls in its older parts. It grows in some cases almost 

 entirelj' submerged in the medium, but in others both submerged and aerial 

 mycelium is produced. Fig. 9, Plate XLIII, shows eight days' growth at 

 room-temperature on ordinary non-neutralized wort gelatine (with which u 

 small amount of well washed, sterilized lamp-black was incorporated), the 

 development of snow-white aerial mycelium being particularly luxuriant. 

 In growths such as this tlie apices of the aerial hyphae are frequently some- 

 what swolleu ; and when such hj'phae are mounted in a drop of water, they 

 burst at their tips, and the contents flow out. The smaller ultimate branches 

 of the mycelium when growing submerged iu a gelatinous or liquid medium, 

 are frequently contorted, swollen, or show various forms of irregularity — a 

 feature common to other species of this genus. 



(6) Sexual Organs. — These were first observed when tlie fungus was grown 

 on sterilized portions of green potato stalks, and they have siuce been 

 observed and studied in detail on various other media. They are produced 

 both on the submerged and on the aerial portions of the mycelium iu sucli 

 media. Cousiderable difficulty was at first experienced in elucidating their 

 structure, and particularly the relationships of the oogonia and autheridia to 

 one another and to the hj'phae bearing them. 



It was found of great assistance first to treat small portions of the 

 mycelium bearing the sexual organs for a few moments with dilute alkali, 

 and, having washed the material free from this, to stain it by immersion in 

 Loeffler's methylene blue, wash again, aud examine iu dilute glycerine. By 

 this means the various stages iu the developmeut of the oogonia aud oospores 

 were made out with considerable facility. 



But much more satisfactory were the continuous observations made on 

 the development of these organs by growing the fungus on thin films of 

 suitable nutrient media, of which extract of oat-agar (see p. 549) was found 

 to be oue of the best. These films were spread as thinly as possible on tlie 

 central portions of the lower surfaces of cover-glasses, wliich were then mounted 

 on deeply excavated microscope slides, in the manner usually adopted for 

 " hanging drop " culture. In the bottom of the excavation a drop of water 

 was placed, and the cover-glass after '' seeding " of the film was sealed to the 

 slide by means of vaseline. This method of procedure offers very considerable 

 advantages over that of the ordinary liquid "hanging drop." If the film be 



