Jan. 20, 1923 
Cytological Study of Infection 
141 
A comparison of the two drawings shows several differences. The 
stoma of Baart is large and opens wide. That of Kanred is relatively 
slender and the aperture is smaller. In fact, the general epidermis of 
Kanred is finer in character, the ordinary epidermal cells being narrower 
than in Early Baart. 
In Baart the fungus enters freely. In Kanred, under the particular 
conditions of these experiments, nine-tenths of the fungi are excluded. 
One would suppose that even the smaller stoma of Kanred, if fully opened, 
would permit the entrance of the fungus. It may be that the naturally 
small stomatal slit in Kanred is still further narrowed by stomatal action 
when an appressorium comes in contact with it. The presence of the 
appressorium might act as a stimulus by mere contact, by altering the 
gaseous exchange through the stoma or disturbing the moisture rela¬ 
tions, by exerting a possible toxic influence upon the guard cells, or by 
its presence shutting off some of the light from the guard cells. It is 
at least conceivable that the guard cells might be sensitive to the appres¬ 
sorium and remain closed, thus excluding the fungus. 
Concerning the effect of light, Pool and McKay (22) have shown in 
connection with their work on the relation of stomatal movement to 
infection by Cercospora beticola Sacc. that light is an important factor in 
stomatal activity, the stomata opening in strong light and closing in 
reduced light. Part of the material from which these counts were made 
was grown in December when the light is poor, and part in March when 
the light here is slightly above average. Both gave an average of about 
10 per cent of entries in Kanred. Mindum and Baart, grown at the 
same times and in the same light and given parallel treatment throughout, 
showed the fungus entering much more freely. If the partial exclusion 
of the fungus is due to deficient light, or to some other factor in the 
environment, it is only in the smaller stomata of Kanred (of the forms 
so far studied) that this factor plays a decisive role. 
In the foregoing studies a strain of wheat stemrust was used to which 
Kanred is extremely resistant. Other strains of the rust are known which 
can attack Kanred and produce pustules. One of these, Puccinia 
graminis tritici III Pers., was kindly supplied by Dr. Stakman, and a 
preliminary study has been made. The seedlings were grown, inoculated, 
and fixed in July, and by good fortune encountered the only hot weather 
of the season. Study of the slides shows that on the whole a larger per¬ 
centage of the fungi enter. There is great variability among the plants, 
an occasional plant showing very few entries, and another as high as 30 
per cent, the average being about 20 per cent. This is based on a study 
of only 8 plants and a total count of about 500 fungi. It remains to be 
determined whether the increase in the percentage is to be explained by 
the fact that host and fungus are congenial or by the fact that in this 
experiment the seedlings were exposed to greater light and heat. 
In the experiments with the rust to which Kanred is resistant, 10 per 
cent of the appressoria effected an entrance. When a Kanred stoma is 
partly closed, the slit is widest near the ends. In the cases of entry in 
Kanred that have been observed, the fungus passes through the stomatal 
slit near one end and swells up inside, forming the usual substomatal 
vesicle, from which a normal infecting hypha grows along the inner sur¬ 
face of the epidermis to the nearest parenchyma cell. Here the hypha 
begins in normal fashion the formation of a haustorium in the mesophyll 
cell. The tip of the hypha forms a broad contact with the host cell, and 
