Dec. aa, 1923 
Cytology of Wheat Stem Rust 
589 
extract but little food from its host, and each attempt to form a haus- 
torium wastes some of its substance, so it soon exhausts itself. 
The number of host cells killed by being entered directly by the fungus 
ranges from one to about twenty or more, a common number being five 
or six. The damage done by the fungus is not limited to this primary 
effect, however. Substances from this “primary area,” if one may so 
designate it, diffuse into neighboring tissues, affecting them to a lesser 
degree. Probably without this secondary damage the fungus would pro¬ 
duce few, if any, flecks, for the patch of dead cells in the primary area 
seldom is large enough to be seen with the naked eye. 
Whether the substances that diffuse outward from invaded tissue are 
the same ones that are excreted by the haustorium into the cell, or others 
f01 med there as a result of their presence, or both, is uncertain. 
It is possible that the fungus does harm in another way. Guard cells 
of stomata occupied by the fungus usually die, although, so far as known, 
the appressorium does not enter those cells to form haustoria. This at 
once suggests the possibility that in later growth portions of living my¬ 
celium in mere contact with the outer surface of mesophyll cells could 
secrete substances that would penetrate the cells and affect their contents. 
Such an effect would be slow and its existence would be hard to prove. 
It would be difficult to find a case where the harm was due unquestion¬ 
ably to this one factor, for a hypha growing out from the center of the 
infection is rarely much in advance of the damage done by the diffusion 
outward of substances from the dead host cells of the primary area. It 
is possible, however, that it does occur and is a minor factor in the situa¬ 
tion. 
These secondary effects of the fungus are not as violent as the primary 
ones, and the tissue so affected differs markedly in appearance. 
One of the secondary effects of the fungus is plasmolysis. Even as 
early as on the fourth day after inoculation (see PI. 3, D) several cells, 
h and i, at some little distance from the fungus, show the cell contents 
shrinking away from the cell wall and drawing together into a ball. 
Very soon after this a layer of tissue, which may be three or four cells 
thick in all directions from the fungus itself, shows decided plasmolysis. 
If this effect reaches the long straight cells of the bundle sheath,which 
seem to serve to some extent for conduction of food materials in the 
leaf, the plasmolyzing agent follows them rapidly for a much greater 
distance, sometimes from one end of the section to the other. At first 
the plastids of the plasmolyzed cells look normal. Later they decrease 
in size. This may be due partly to starvation, as the shrunken cell con¬ 
tents have lost normal relations to other cells. Still later there is a 
partial or complete recovery from plasmolysis in the outer part of the 
affected zone, which now betrays its former trouble only by the minute¬ 
ness of its plastids and the abnormal condition of a few of its nuclei. 
The tissue nearer the source of trouble also may recover if not too severely 
affected, but more commonly cells close to the primary area are quite 
empty and perfectly clear, although retaining their original shape. 
Another secondary effect of the fungus is a swelling of the host cell 
walls. This occurs irregularly, here and there, never affecting all of 
the cells within the influence of any one fungus, nor even all of the wall 
of any one cell. Moreover, one infection differs markedly from another 
in this respect. 
