A Dry Rot of the Irish Potato Tuber 
43 
terial. In fact, variation between plates grown under the 
same condition may be greater than that between plates grown 
under different conditions. This does not hold for all species of 
Fusarium by any means, as shown by the Fusarium which we 
have found on Colorado potatoes. In the light, this fungus 
assumes a pink color; in the dark, a white, soon followed by a 
deep blue. 
The plates placed in bright light so that the sun struck them 
during a part of the day, and so that the temperature rose to 
30° to 40° 0., showed the most intense color, but a slow growth. 
Here the color very often went over to the salmon tint, page 72 
of the Repertoire, especially on the underside of the culture. 
The question whether the colony will appear flesh-pink or pale- 
pink is largely a matter of secondary germination of the conidia. 
If these germinate on the original mat and the nutrients become 
exhausted before the second set of hyphae produce conidia, the 
mycelium color will tone the conidial color down to a pale pink. 
Conidia are produced more slowly in such cultures and the 
mycelial growth predominates. The growth here is rapid and 
the media are not exhausted completely as the fungus advances; 
consequently a secondary luxuriant growth takes place over the 
first layer. In bright light the growth is slower, more conidia 
are formed, and the mycelium uses up the food completely; 
consequently less secondary germination takes place and there- 
fore the original pink color is not obscured. The catalytic 
effect of the sun's rays may have an influence fostering a more 
complete respiration of the media as the fungus grows along. 
The cultures in flasks on liquid media gave contradictory 
results. The fungus when grown in distilled water and when 
grown in dark and in light develops a thin mycelium which shows 
a lilac mauve color, Repertoire, p. 196, No. 1. When grown in tap 
water the fungus grows very sparingly and develops no color at 
all. On 1 per cent glucose in light the fungus develops a color, 
Repertoire, p. 135, No. 3, when two or three weeks old and no 
surface growth occurs and no conidia are formed. In the dark 
the growth is very stunted and the color is white. Microscop- 
ically no difference, excepting that the fungus grown in the dark 
showed a very knotted appearance, is apparent. When grown 
on culture media in the light a very luxuriant growth of white 
mycelium takes place and no color is developed until the fungus 
reaches the surface and begins to form a mat of plectenchyma 
which soon becomes covered with innumerable new conidia. At 
no time does there develop any color in the mycelium in the 
medium. 
When grown in the natural habitat, the potato away from 
