Jan. 20, 1923 
Cytological Study of Infection 
*45 
but is still plump. At a is an empty haustorium mother cell still con¬ 
nected with b, the large worm-shaped haustorium produced by it. At 
c is a second empty cell and its haustorium. A considerable part of the 
contents of the host cell has collapsed into a mass about these two 
haustoria. The remainder of the cell looks normal. The host nucleus 
which is included in this shrunken mass shows decided signs of disinte¬ 
gration. Although this part of the host cell has collapsed from the com¬ 
bined effect of two large haustoria, the fungus itself does not seem to be 
harmed. The haustoria have not collapsed, nor is there any discoloration 
of the hyphae outside. 
The results of a somewhat different balance of forces is seen in Plate 
6 , C, which represents a cell of the same mycelium but two or three cells 
away from those drawn in B. Here the host cell has not begun to col¬ 
lapse and shows no signs of deterioration due to the fungus, but the 
haustorium at a, which was evidently very large, has collapsed into a 
crumpled, irregular mass; the haustorium mother cell outside at b is 
wrinkled, flattened, and discolored, and the hypha below it is similarly 
affected for a short distance. 
The details of the process may vary in different cases, but they come 
to the same end. Plate 6, D, is typical, with the dead host cell e stained 
a dense red and showing but little or no detail in cell contents, the col¬ 
lapsed haustorium mother cell a with its walls stained deep red, and the 
hypha devoid of cytoplasm below this cell and also for some distance 
up the branch springing from its base. 
Although the reaction by which host and fungus are killed is consid¬ 
erably slower here than in the cases described earlier, it is still fairly 
rapid. Another mycelium, of the same age and found in the same leaf 
near this one, was completely killed. The dead host cells and an occa¬ 
sional dead hypha were all that remained. 
In older material, all traces of the mycelium disappear except the initial 
hypha at the stoma. Just how this happens is not known. The hyphae 
can hardly be supposed to dry up, for the intercellular spaces of the leaf 
must form an excellent damp chamber. Moreover, the hypha nearest 
the stoma where some drying might take place is the one that persists 
the longest. In material taken 10 days after inoculation, no traces of 
intercellular mycelium were discovered. 
DISCUSSION 
Pole Evans (13 ), in a comparative study of the histology of the uredo 
mycelia of cereal rusts, notes characteristic differences between the species 
and tabulates his results as to the appressorium; the form, size, and septa- 
tion of the substomatal vesicle; the number of the infecting hyphae; the 
size and form of the haustoria, etc. He says: 
These different species of Puccinia in the early stages of development of their uredo 
mycelia exhibit morphological characters (seen especially in connection with the 
formation of the substomatal vesicle) which serve at once to distinguish them from 
one another. 
There doubtless are typical morphological differences between the vegeta¬ 
tive mycelia of different rusts, but Plate 1, F and G, shows some of the 
difficulties involved in any attempt to classify rusts on this basis. We 
have here the same rust on the same host. Yet in one case there is prac¬ 
tically no vesicle and only one infecting hypha, and in another there are 
a large vesicle and four hyphae. All gradations between the two can be 
