BLACK ROT OF THE POTATO I MORPHOLOGY 269 



does motility persist in cultures?), presence and distribution of 

 flagellse (use Hugh Williams' stain), absence of endospores (try 

 heating for 10 minutes at 80C., and spore stains). Any 

 capsule? Reaction to Gram's stain (examine with the dia- 

 phragm wide open). Is the organism acid-fast? Do involution 

 forms occur? Test in dextrose peptone water. 



2. Cultural Characters. Growth on very thin-sown -f-10 

 beef-peptone gelatin plates at 20C. is very characteristic 

 (If the plate contains only 4 to 6 bacteria, they should grow 

 quickly, forming big circular opaque white colonies (Fig. 206) in 

 3 or 4 days. Compare with No. VI (Fig. 212A.) The rate of 



FIG. 205. a. 48-hour gelatin colonies of Bacillus carotovorus L. R. Jones, 

 grown at same temperature and on same batch of gelatin as b. Natural size. 



b. Gelatin colonies of Bacillus phytophthorus Appel. Photographed natural 

 size after 48 hours at 18C. 



liquefaction in gelatin stabs resembles that of Bacillus carotovorus. 

 Appearance in agar plates, streaks and stabs. Growth on 

 steamed potato. Behavior on slices of raw potato (select 

 smooth, sound tubers, w r ash, soak 40 minutes in 1 :1000 mercuric 

 chlorid water, slice when dry with a sterile knife, and put into 

 shallow-covered culture dishes or deep Petri dishes) . Why does 

 the black stain not appear also in the cultures on the cooked 

 potato? What is tyrosinase? Does it play any part here? 

 Why does the dark stain disappear in later stages of the rot in 

 raw tubers (Fig. 201)? Growth in bouillon, nitrate bouillon, 

 Cohn's solution, Uschinsky's solution, Fermi's solution (Does 

 this medium become yellowish or greenish?). Growth in milk 

 (Is the casein thrown out of solution by an acid or by a lab 

 ferment? What is the odor of the fermented milk? Does this 



