596 
Journal of Agricultural Research 
Vol. XXVI, No. ia 
main closed; or a loss of turgor in the cell contents that also would prevent 
the opening of the stoma. Later on, the guard cell walls might be. so 
softened that the fungus could push through without the normal opening 
of the stoma. The last seems least likely, as entries usually take place 
before the stomata are so far deteriorated. The majority of the fungi 
enter, except in Kanred, and the guard cells die in much the same way 
when the fungus does enter as when it does not. Perhaps the entry of 
the fungus depends on its promptness. It may be that the fungus must 
enter before it destroys power of motion in the guard cells. 
Loftfield (22), in his studies on the behavior of stomata, found that 
stomata of cereals are closed at night, open partly (only 30 per cent of 
the maximum aperture) in the morning, close before noon, and remain 
closed until the next morning. Stomata of wheat plants grown in the 
greenhouse open wider and remain open longer than those of plants 
grown out of doors. He notes that cool and rather humid weather and 
less sunshine favor the opening of wheat stomata, and points out the 
relation of this to the spread of wheat rust. Cereal stomata rarely 
show the maximum opening, and many are closed even during the 
morning. He says ( 22 , p . 40 ): 
It is not definitely known whether a few stomata with more accessible water supply 
do the opening on days of unfavorable conditions, or whether groups of stomata open 
and shut very rapidly and at different times. Direct observations on the same leaf 
would indicate the former, but the fact that open and closed stomata occur in groups, 
and that the stomata of cereals can open and close with amazing rapidity, makes the 
latter hypothesis possible. 
And ( p. 45)— 
In all cereals the tendency seems to be to operate with many closed stomata at all 
times. 
This tendency in wheat to keep many of its stomata closed may ex¬ 
plain the fact that some apparently vigorous appressoria remain outside. 
If the stoma remains closed for two or three days, the secretion by the 
appressorium probably would render the mechanism of the stoma inop¬ 
erative. It would lose power to open and the fungus would be excluded. 
The fact that open and closed stomata are found in groups may explain 
the differences in the percentage of entries in different parts of the same 
leaf. In Kanred, with its smaller stomata, these peculiarities in stomatal 
behavior might result in the exclusion of a much larger percentage of 
the fungi than in varieties with large stomata. 
In Mindum, the host in which the effect of the fungus on the stoma is 
most pronounced, portions of the mesophyll cell walls in infected areas 
undergo marked swelling. No such result was seen when this rust grew 
on the other hosts. The difference, whatever it be, lies in Mindum. 
As noted before, these swellings do not occur, so far as can be dis¬ 
cerned, in the walls of host cells whose death is most violent. In other 
words, where the substances secreted by the fungus are most concen¬ 
trated, and the host cells are killed rapidly, the swelling of walls does 
not occur, or at least it can not be recognized. Perhaps other substances 
formed during the death of the host cell inhibit the reaction. The 
swollen walls are seen best farther out where the host cells are still 
living, although somewhat harmed by the diluted secretions that have 
diffused out to them, or in older infections where the reaction is milder. 
It is not clear whether this means simply that the substance secreted by 
the fungus is free to act there, or that the diluted secretion is best suited 
to the purpose, or that the active participation of a living host protoplast 
is necessary to produce the swelling. 
