Dec. 22 ,1923 
Cytology of Wheat Stem Rust 
573 
Seedlings were grown in the greenhouse in 4-inch pots. About 10 days 
after sowing, the first leaf of each seedling was inoculated, the plants 
placed in damp chambers 48 hours and then kept in cheese-cloth cages. 
Material was fixed daily from the second to the fifteenth day after inocu¬ 
lation and, for comparison, different lots were grown and fixed at dif¬ 
ferent times of the year. 
On the whole, the best fixation was obtained with the standard solu¬ 
tion: 1 gram chromic acid, 1 gram acetic acid, % gram urea, in 100 cc. 
of distilled water. The chief trouble in fixation seems to be due to slow 
penetration. The majority of the stomata in wheat are closed, and the 
rest of them close partly or completely in the fixing fluid. The air im¬ 
prisoned in the large intercellular spaces of the leaf is displaced but slowly, 
if at all, by the fixing fluid. The fluid works its way in only through the 
cut surfaces at the ends of the piece. The spongy mesophyll tissue com¬ 
posing the greater part of the interior of the leaf is made of lobed, irregular 
cells having small surfaces of contact with each other. Penetration from 
one to another of these cells is slow, and the central tissues of a piece of 
leaf deteriorate, especially in a warm room, before the fixing fluid reaches 
them. This trouble was largely obviated by placing material during 
fixation in a cold-storage chamber at 42 0 F., which preserved the tissues 
during penetration. The tissues were embedded in paraffin, sectioned, 
and stained with Flemming's triple stain. 
BAART AND PUCCINIA GRAMINIS TRITICI FORM III 
MACROSCOPIC OBSERVATIONS 
Form III of Puccinia graminis tritici Erikss. and Henn. produces the 
4— or 4 type of infection on seedlings of Baart wheat. It develops 
vigorously and gives evidence of being on a congenial host. The flecks 
or discolorations of the leaf marking the location of the young infections 
make their first appearance about the sixth day after inoculation. These 
are ellipsoidal in shape, a uniform light green in color, and about a milli¬ 
meter in diameter. The flecks grow, and on the eighth or ninth day the 
fungus breaks through the epidermis at the center of each spot, forming 
the open uredinium. At maturity, about two weeks after inoculation, 
the uredinia, if isolated, are 3 or 4 mm. long and about half as wide and 
are surrounded by a zone of paler tissue 1 to 2 mm. wide. This discolored 
area is not white or gray or yellow, but is green and only a shade or so 
lighter than the rest of the leaf. When uredinia are crowded together, 
they are much smaller and the discoloration of the leaf may be continuous. 
MICROSCOPIC OBSERVATIONS 
An earlier paper (2) reports a cytological study of Baart infected with 
Puccinia graminis tritici form I and another strain of stem rust found in 
Berkeley and since identified as form XIX. Baart is susceptible to both 
of these forms of the rust, and their history was followed in some detail 
through the first week of development, giving the formation of the ap- 
pressorium upon the stoma, its entry through the stomatal slit, the forma¬ 
tion of the substomatal vesicle, the growth of the infecting hyphse from it, 
the formation of haustorium mother cells, and the development of 
haustoria. Puccinia graminis tritici III grows equally freely and normally 
on Baart. A description of the first week of its development would be 
in large measure a duplication of what already has been written, and so 
is not repeated here. 
