Jan. 20,1923 
Cytological Study of Infection 
H3 
# Plate 4, E, a tangential section of material taken 10 days after inocula¬ 
tion, shows the final stage of this process. The fungus evidently entered 
several days before the material was fixed. At a is seen a cross section of 
the projection from the appressorium that pushed through the stoma, and 
at b the hypha is seen. The part connecting a and b was cut off in sec¬ 
tioning. The hypha grew to the nearest mesophyll cell. At its distal 
end this hypha fades out and seems to be partially dissolved. Only one 
host cell was killed, and it is pale, having lost the intense affinity for stains 
possessed by newly killed cells. No structures are recognizable within the 
cell, and it is shrunken and no longer jagged in outline. A markedly 
thickened wall is to be seen at the contact between the dead cell and the 
healthy mesophyll cell adjoining it. 
A fungus which has been checked in its first attempt to establish par¬ 
asitic relations with the host may still possess enough vigor to grow. In 
Plate 4, D, the nucleus and cytoplasm certainly suffice for the production 
of a secondary infecting hypha and cases are to be found in which this 
has happened. Plate 5, A, is a section nearly tangential, showing the 
stoma and adjoining epidermis above and mesophyll tissues below. The 
fungus entered the stoma at c and formed two infecting hyphae. One at 
e attacked a mesophyll cell with the result that both the cell and the 
hypha leading to it died. Several slender threadlike projections on this 
dead host cell evidently represent the remnant of the irregular, jagged 
points seen on attacked mesophyll cells in earlier stages. A disintegrat¬ 
ing remnant of the haustorium mother-cell is to be seen at the left of the 
dead cell near e. A shorter infecting hypha was formed at 6. The killed 
host cell lies just above it and is found in the next section. These two 
attempts did not exhaust the fungus. A third hypha pushed out running 
along the edge of the stoma to a, and its tip some distance farther on 
contains a meager amount of living cytoplasm. The fourth and last 
attempt is at d, where a minute haustorium mother-cell is forming, 
equipped with thin cytoplasm and two nuclei so small and faint that they 
are scarcely distinguishable. 
An extreme case of this sort is seen in Plate 5, B, where a fungus made 
no less than six distinct attacks. Two fungi entered this stoma, as may 
be seen from the two well-developed substomatal vesicles. Of these, 
one was quickly dispatched. A single infecting hypha was formed, 
resulting in a dead mesophyll cell at b and a dead discolored hypha at a. 
The other pushed off two primary infecting hyphae, numbers 6 and 1. 
With the death of No. 1, No. 2, a smaller, less vigorous hypha, branched 
out from its base. No. 2, in turn, died, and then came successively 
smaller and feebler hyphae, numbered 3, 4, and 5. Only the last two 
of these contain living cytoplasm. Nuclei, if present, are too small to 
be definitely distinguished. One can hardly believe that a single spore 
could provide sustenance for so long a struggle. It is possible that the 
same cytoplasm, retreating from one infecting hypha, takes part in the 
next attack. It is to be noted that there is a septum near the base of 
each one of these dead infecting hyphae which might mark the limit of 
the retreat as did the septum at c in Plate 4, D. It may also be that the 
reaction of the host against the fungus was a little slower in this case and 
that some slight nourishment was extracted by the fungus before the 
toxic reaction began. 
Several of the host cells in this preparation have completely collapsed 
and others are dying. The walls of the latter are thick and stained red. 
