177 
Potato Scurf and Potato Scab. 
dissolve the starch grains in the ordinary way, but all cell walls 
had entirely disappeared, and the starch grains formed a plaster- 
of-Paris-like spongy lump. When dry, this lump could be 
rubbed easily to a powder between the fingers, and even without 
magnification was seen to be pure starch. Inside, the lump was 
quite white and mealy ; towards the outside it was yellowish, 
becoming green at the exterior. Microscopically examined, the 
white mealy portions consisted of sound starch grains ; the 
yellowish, and especially the greenish portions, consisted also 
of starch grains, but they had been curiously attacked, being 
pierced by the mycelium (see Fig. 3, p). In some, the 
mycelium had passed right through them ; in others it pene- 
trated the interior, giving off short branches ; whilst others 
were entirely filled with it, but their outlines had not been 
altered. Phizoctonia thus differs from the other known fungi, 
which dissolve and absorb the starch in the cells of the host 
plant. The affinities of this fungus in its treatment of the host 
plant may be found with those fungi occurring on timber. 
The furrows on the epidermis caused by the mycelium burst 
open in wet seasons, and deep fissures result, through which 
the mycelium may have access to the interior of the potato, as 
shown in Fig. 3, G. In the early stages where the furrows 
are slight the potatoes are marketable, and their nutritious 
value is not impaired ; the tubers are rendered, however, unfit 
for the market when exhibiting the deep fissures. 
The following conclusions result from this investigation : — 
1. The scurf appearance does no injury to the potato, either 
as an article of food or for use as seed. 
2. The scab disease in its earlier stages does not affect the 
food value of the potato, but in no case should the tubers 
be used as seed. 
3. The so-called deep scab is only a further stage of the 
scab disease, and is of less value as food material and more 
dangerous if used as seed. 
Hans Th. Gussow. 
The Laboratory, 
44 Central Hill. Norwood. S.E. 
