250 The Potato Disease. 
potato on February 4th; the day following the tubes of the 
mycelium had penetrated the leaves ; on the 8th the mould ap- 
peared on the under surface covered with fruit, and on the 9th 
the whole plant was diseased. 
Fig. 3. — Spore-hearing mould, Peronospora infestans. 
Olagnified 300 diameters.1 
The spore-bearing mould springs from the mycelium, which penetrates the 
tissues of the leaf, and passes through the stomatcs on the under surface into 
the air. 
The individual cells which are pierced by the mycelium are 
destroyed, and the starch-granules contained in the cells are 
attacked and consumed. Putrefaction soon begins, affecting first 
the cell-walls and then the starch. Payen has put it beyond 
doubt that the mycelium consumes the starch, for in his investi- 
gations he detected the granules attacked by the mycelium 
threads, and he made the injury more apparent by using iodine, 
the action of which on colouring starch granules is well known. 
By the ordinary processes all the starch can be separated from 
diseased potatoes, not only that contained in cells yet untouched 
by the mycelium, but even the granules that remain uninjured by 
the mycelium or the surrounding putrefaction. 
