I'ethybkidgk and Laffkrty — Dry-Rot of the Potato Tuber. 207 



best of those used were Quaker Oat agar, oat extract agar, 1 slices of cooked 

 potato tuber, and cooked potato stalks. In all cases where gelatine media 

 were used, the fungus caused liquefaction of the gelatine. On the four media 

 mentioned the fungus grew well, developed both conidia and chlamydospores, 

 and exhibited the characteristic blue coloration which its specific name 

 implies. When grown on ordinary beef-extract agar or gelatine, or on 

 potato-juice gelatine, the colour was not produced, and chlamydospores were 

 absent. The colour was, however, produced in cultures on beer-wort gelatine, 

 but here, again, no chlamydospores were developed. 



On Quaker Oat agar the growth is at first white. Later on the medium 

 which the mycelium permeates becomes claret-coloured, while the aerial 

 mycelium shows all stages of colour, from white to claret and blue. The 

 conidia are borne in more or less slimy or viscous masses on the surface of 

 the medium, and these masses vary in colour, from dirty white, through 

 cream, to light green aud light blue. On this medium isolated, hard masses 

 of compacted, blue mycelium, of a sclerotial nature, are produced in con- 

 siderable numbers. On oat extract agar growth is similar, but rather less 

 luxuriant. 



On slices of cooked potato tuber the mycelium produces a distinct blue 

 colour in the medium, while the moist masses of the conidia on the surface 

 are coloured distinctly light green. Chlamydospores are produced abundantly, 

 and so are the blue sclerotium-like bodies already alluded to. 



On cooked potato-stalks the growth is very similar. Chlamydospores 

 are present, but the blue sclerotium-like bodies are not developed. 



Influence of Light, — Our cultures were carried out in test tubes 15 cm. 

 long, which for convenience sake were grouped in tens, and were placed in 

 round tins about 8 cm. high. Even when standing in the laboratory the 

 amount of light which could reach the culture was therefore but small ; and 

 when, as was often the case, they were kept in a dark cupboard, light was, 

 of course, totally excluded. Under these circumstances, the substratum and 

 mycelium were as a general rule of a more or less indigo blue colour, while 

 the superficial moist conidial masses were coloured light green. 



Parallel cultures were carried out in light and in darkness to ascertain 

 what influence (if any) light had on the coloration produced. The cultures 

 were kept in a window facing north, so that they were not exposed to direct 

 sunlight. One half of them were fully exposed and the other half were 

 carefully covered with thick black paper in such a way as to exclude all 



1 For the method of preparing these media see Sci. Proc. Roy. Dublin Society, xiii, 

 (N.S.). No. 36. March 1913, p. 578 and p. 580. 



