DCC. 22, 1923 
Cytology of Wheat Stem Rust 
601 
outright. There is widespread plasmolysis, the nuclei collapse sharply, 
and the plastids disappear. Farther away, where the secretion of the 
fungus is more dilute, the host nuclei live and even undergo slight 
expansion. 
It would seem, then, that the fungus in some fashion stimulates its 
host to greater metabolic activity; that the enlargement of the nucleus 
is an index of this increased activity; that where the nuclei enlarge most, 
the cell, and particularly the plastids, are least impoverished; and that 
the metabolic products of this heightened activity help to meet the 
needs of the fungus. 
SUMMARY 
Baart is susceptible to specialized forms III and XIX of Puccinia 
graminis tritici; Kanred is susceptible to form III and immune from 
form XIX; and Mindum is immune from form III and resistant to 
form XIX. 
Appressoria of both forms of rust secrete some substance which pene¬ 
trates the walls of the guard cells on which they lie and spreads through 
them, sometimes reaching the next cells. This substance softens the cell 
walls and kills the cell contents. It produces the minimum of injury to 
the stomata of Baart, is intermediate in its effect on Kanred, and 
strongest in its action on Mindum. 
This effect upon the stomata seems to be independent of susceptibility 
and immunity, for the two forms are nearly equal in their effect on the 
stomata of any given host, yet differ markedly from each other in their 
ability to infect these hosts. 
This secretion by the appressoria causes a softening of the guard cell 
walls and the death of the cell contents. With either of these effects 
the mechanism of the stoma presumably would fail and the stoma would 
remain closed. The entry of the fungus usually takes place before this 
alteration of the guard cells becomes pronounced. 
The percentage of entries in Kanred varied from 5 to 18, with a general 
average of 13. In the other hosts it ranged from 23 to 78. 
The peculiar behavior of wheat stomata, as observed by Loftfield, 
probably explains to some extent the partial exclusion of the fungus. 
Form III develops normal haustoria in Baart and Kanred and obtains 
food for growth. Where the mycelium is densest, its demands upon the 
host for food are greatest, and its secretions into the host cells are most 
concentrated, the host cells are stimulated to increased metabolic ac¬ 
tivity. The nuclei increase in volume several fold. The plastids first 
decrease in size, but with the increase in the activities of the cell, this 
reduction is checked and a balance of forces is struck and maintained. 
In the outlying regions of the infection in Baart the stimulus comes 
later and is weaker. At first the activity of the host cells is not increased, 
their nuclei do not expand, and the reduction in the size of their plastids 
is not checked. Consequently, the plastids become far smaller than 
those at the center of the same infection. Later there is an expansion 
of the nuclei in these outer regions and a partial recovery in the size of 
the plastids. In Kanred the marginal regions are stimulated sooner 
and the reduction in plastid size is less extreme. 
Still later the host nuclei throughout the infected area collapse. 
Form III also forms haustoria in Mindum. When the young fungus 
forms a haustorium in a mesophyll cell, the living contents of that cell 
flow rapidly to the haustorium, condense around it, and die, and the 
