4 BULLETIN" 82, U. S. DEPARTMENT OE AGRICULTURE. 
It seems established that the amoebae pass from one ceil to another 
when cell division takes place, although it is claimed by Massee (1908) 
that the amoebae invade new cells by boring through the cell walls. 
On the other hand, Osborn (1911), who has made an extensive cytolog- 
ic al study of this organism, holds that "on the division of the host 
ce ]| * * * \i j s a purely fortuitous circumstance whether each 
resulting cell shall contain an amoeba, and so be infected or not. 
* * * I have never seen any signs of the migration of an amoeba 
to a neighboring ceil nor any continuity of protoplasm, such as 
Massee has described." Osborn's contention as to the method of 
migration of the amoebae has been confirmed by Home (1911). The 
abnormal local increase of cells causes a swelling and a faint discolor- 
ation of the skin, which latter becomes a wartlike outgrowth. The 
fungus present in this tissue consumes largely the contents of the 
cells (PI. I, C, B), after which the amoebae coalesce (PI. I, C, D) and 
form one or more large spongy masses in each cell, known as plasmodia 
(Pi. I, D) . These latter divide into many small spores, each of which 
takes on a heavy yellowish brown wall (PI, I, E). Instead of these 
spores separating, they remain attached, forming a l> spongelike body," 
according to Johnson (1908), and not a hollow sphere, as reported by 
Berkeley (1846) and Massee (1910). 
Since the contents of the attacked host cells are used up in form- 
ing spore balls, the infected area becomes a pit filled with a yellow- 
ish brown dust consisting chiefly of spore balls (PL II, C). These 
pits, or sori, at maturity are bordered by the torn skin of the tuber. 
The torn skin standing up on the periphery of the sorus is one of the 
characteristics of powdery scab, which, often enables one to dis- 
tinguish it from the Oospora scab macroscopically. The powdery con- 
tents of the sori in this stage of the disease doubtless suggested to 
Johnson the name "powdery scab/' as has already been noted. It 
has been observed that in storage shriveling and shrinkage take place 
about these pits, or sori. How generally this shriveling occurs and its 
significance are not known up to the present writing, nor have these 
matters been emphasized in the literature, to the writer's knowledge. 
The cause of this shrinkage is not known, but it may possibly be due 
to insufficient cork deposition in the bottom of the sori which afford 
an avenue for storage rots to attack the infected tuber. 
When conditions are highly favorable for the fungus, it may eat 
large cavities in the immature tubers. Besides consuming part of 
the tubers, it stunts their further growth and produces malformed 
tubers, such as are shown in Plate III. The nature and extent of the 
depressions caused are shown in Plate III, C. This stage of the dis- 
ease has been called the cankerous stage (Home, 1911) and is the 
one that causes the greatest loss. 
