121 
a portion of the diseased part will show that the decay commenced 
where the vascular bundles concentrate. See 2, Fig. 1. ° At that point 
the air is in greater volume than elsewhere. 
i Ai IA 
At AVES 
\\ A \ 
AIW 
2 
When such spots are exposed to the atmosphere the fungus, blue 
mold, forms on the surface. This disease, therefore, has no relation to 
potato-rot, as ordinarily understood. F represents the mycelium (spawn) 
of blue mold, Pennicillium glaucum, and G that of potato-rot, Peronospora 
infestans. 
The vascular bundles are much smaller in some varieties of pototoes 
than in others, and the texture of the cellular matter varies also. I 
think it probable that those varieties having the smallest air-passages, 
all other considerations being equal, will be the least affected by the 
fungus Peronospora infestans. 
The air passages of the vascular bundles may be easily seen by the 
