288 
Journal of Agricultural Research 
Vol. XXIII, No. 4 
MICROSCOPIC CONDITIONS 
Kunkel (j) has observed the presence of the Spongospora plasmodia 
in the shrunken areas around the old sori. These the writer has not 
been able to detect in his sections of the skinspot pustules. The mature 
spore balls, naturally, are not to be found in the immature sori. Wol- 
lenweber ( io ) states that in his material “Spongospora was absent alto¬ 
gether.’* He does not explain what particular stage of Spongospora was 
sought in the “pustelfaule” spots or by what methods its absence was 
ascertained. The difficulty of demonstrating the plasmodium in the 
pustules of skinspot may find its explanation in the fact that at certain 
stages of its development it may, apparently, disappear from the spaces 
it has formerly occupied. This phenomenon has been noted by Kunkel 
(j) even in very young sori. He states : 
Just beneath the epidermis can be seen the dark intercellular spaces which were 
previously occupied by the infecting plasmodium. 
Rig. i.—S emidiagrammatic drawing of a section through a typical skinspot pustule, showing the tapering 
spread of the infection between the cells. In several cases the infection has penetrated into the cells, 
A mass of broken-down cell walls is shown in the center of the spot. Some cells are slightly enlarged. 
The point marked by the arrow is probably the place of origin of infection. No hyphae have been 
observed in this spot. 
(See also his plates XXXIX, XU, and XUI.) Aside from this the 
general appearance of the vertical sections through the raised pimples of 
skinspot is very characteristic of the condition within the closed sorus 
of powdery scab (fig. i). The center is usually filled with a disorganized 
mass of broken-down cell walls. This debris is surrounded on all sides 
by darkened intercellular spaces, indicating the path of infection. The 
infection spreads between the cells in the form of characteristic tapering 
projections. Occasionally a portion of the cell wall is destroyed and the 
infection extends into the cell. In some sections hypertrophied cells 
may also be observed. Fungus hyphae are entirely absent in a certain 
number of the pustules, which is fully in accord with the results of isola¬ 
tions (Table i). Such spots may occasionally yield bacteria, but often 
when transferred to plates they remain sterile. In certain other spots 
the mycelium, although present, may be found only in very small 
quantity, dissociated from the apparent course of infection and obviously 
out of proportion to the area presumably killed by this mycelium. 
