Pkthybridge — On the Rotting of Potato Tubers. 537 



exposed to the air they became pink, and later on they blackened in the 

 normal manner. The depth to whioli the blackening extends below the cut 

 surface is not very great : in one case in which measurements were made it 

 was found to be only about 2 mm. after 2g days. When this darkened layer 

 was removed, the freshly cut surface, on exposure to air, exhibited the same 

 series of colour changes as before. The change of colour seems to be accom- 

 panied by a change in the reaction of the juice in the dead cells, for whereas 

 at first the sap is quite acid, after blackening has occurred it is distinctly 

 alkaline to litmus. This alkalinity becomes more pronounced later on when 

 bacteria, wliich develop upon the exposed cut surfaces, inaugurate further 

 chemical changes. 



The changes mentioned here may recall somewhat similar ones described 

 in tlie case of tubers affected with the organism causing Black Stalk Rot, 

 Bacillus melanogenes P. & M. But in the case of Pink Eot the changes 

 in colour are much more definite and pronounced than in Black Stalk Eot. 

 In any case there are two important characters which enable these two 

 diseases to be distinguished from one another with ease. B. melanogenes 

 destroys the middle lamella of the cells, and causes a distinct maceration of 

 the tissues into a soft pulpy mass, and their consequent shrinkage results in 

 the production of one or more cavities in the rotting tubers. In the ease of 

 Pink Hot no maceration occurs (at any rate not until the already killed 

 tissues have been invaded by extraneous organisms), and, far from being 

 rendered pulpy, the tissues become tough, leathery, or almost rubber-like in 

 texture, this probably being caused by the fact that the intercellular 

 mycelium of the fungus binds the no longer turgid, dead cells together in its 

 meshes. Not a single tuber affected with Pink Eot has as yet been found 

 witli a cavity within it, except at a period long after the whole tuber has 

 been killed, and its dead tissues liave become the seat of secondary changes. 

 Cavities are common, in fact almost the rule, in tubers suffering from tlie 

 attacks of B. melanogenes, even at a period considerably before tlie whole 

 tuber becomes diseased. 



With regard to the microscopical appearances presented by the diseased 

 tissues, it may be stated here that sections made preferably near the 

 junction of healthy and diseased cells show an abundance of rather wide, 

 much-branched non-septate hyphae, rich in protoplasm, running, as far as 

 could be made out, exclusively between the cells. No haustoria were observed 

 entering the cells but special efforts to prove their presence or absence were 

 not made. The starch grains appear to remain absolutely intact. 



Although prolonged and repeated attempts have been made to discover 

 the reproductive organs of the fungus in or on tubers, none of any kind have 



PCIENT. PROC. R.D.S., VOL. XIII., NO. XXXV, 4 I, 



