208 Scientific Proceedings , Royal Dublin Society. 



light. The inoculum used in each case consisted of a portion of a green, moist 

 mass of conidia, together with a small amount of adhering mycelium. 



The experiment lasted forty days. In the dark the conidial masses were, 

 as a rule, of the greenish blue colour already familiar to us, with occasionally 

 some which were white or dirty white in colour. In the light, however, the 

 conidial masses were of a " bitter-sweet pink " (Eidgeway) colour. 



Influence of Moisture. — This experiment was repeated in a slightly 

 modified form. Not only were the cultures divided between light and 

 darkness, but the effect of an abundant and of a restricted amount of moisture 

 was studied at the same time. With regard to the latter, no effect was 

 observable as far as development of colour was concerned, but the extra 

 moisture merely led to a more luxuriant growth of the culture. As in the 

 previous experiment, however, it was found that the presence of light caused 

 the development of the bitter-sweet pink colour in the masses of conidia, 

 while in the absence of it they were light green. It has already been pointed 

 out that the fungus pustules present on rotting tubers become pink in colour 

 when exposed to a good light. 



Appel and Wollenweber describe the pure colour of the conidia of 

 F. cceruleum as being reddish-ochre when grown in bright daylight, but 

 brownish white with a minimum of light. The verdigris-green colour often 

 observed in the conidial masses is regarded by these authors as being a 

 mixture of the brownish-white of the conidia with the indigo blue of the 

 substratum. We, however, were not able to confirm this. 



Nature of the Colouring Matter. — If a portion of a culture in which the 

 blue colour is well developed (as, for instance, one on cooked potato) be boiled 

 in water, no extraction takes place. The colour, however, is extracted by 

 dilute hydrochloric acid in the cold, and it at once changes to pink. The 

 addition of an alkali to the pink solution causes it to become blue. 



Influence of Acid or Alkaline Reaction of Media. — Cultures were carried 

 out on media, made both distinctly alkaline and distinctly acid, and in the 

 former case the colour developed was deep blue. In the acid medium the 

 fungus grew satisfactorily, but the colour first produced was of a wine- red 

 tint. Subsequently, when the acidity of the medium had become neutralized 

 by the alkalinity, developed as the result of the growth of the fungus, 

 the colour changed to blue. It may be remarked here that the juice 

 expressed from the diseased portion of an affected tuber gives an alkaline 

 reaction to litmus, while the norm al juice of a healthy tuber is, of course, 

 slightly acid. 



Proofs of Pathogenicity. — A large number of experiments involving the 

 inoculation of healthy potato tubers with the various strains of the fungus 



