Jan. 20, 1923 
Cytological Study of Infection 
l 31 
A hypha about to form a haustorium undergoes certain very charac¬ 
teristic changes, and as these cells are to be found by hundreds there is 
little doubt as to the details of the process. In Plate 2, B, the hypha, 
in growing up along the surface of one mesophyll cell, touched a second 
mesophyll cell obliquely at b, became deflected, and then the tip struck 
a third cell squarely at c. Growth in length is forcibly halted, the hypha 
has thickened, its contents are concentrated near the tip, and the two 
nuclei have moved up. 
The next step in this process is seen in Plate 2, C. The hypha growing 
up touched a host cell first on one side, then on the other, and then the 
end struck a third cell at a. There was room enough to grow on, by 
bending, but growth in length was checked temporarily and other changes 
started. The slender-tipped growing hypha has now swollen into a 
broad club with its end closely applied to the wall of the host cell. The 
pair of nuclei which lag behind in a rapidly growing tip have moved up 
and have evidently undergone division. Two of the daughter nuclei 
have moved out into the swollen terminal portion. The other two are 
lying a short distance behind. The four daughter nuclei are nearly 
full grown and are about equal in size. 
Soon after this a septum forms, thus giving rise to a short terminal 
cell. This is the haustorium mother cell. Plate 2, D, shows such a 
case and also A at b and e and C at b. This haustorium mother cell 
contains one of the two pairs of daughter nuclei. It varies considerably 
in shape according to the available space, but when relatively free it is 
oval. 
There has been a marked change in the size of the nuclei. Before 
the septum formed, all four were approximately equal. Now the pair 
of nuclei in the penultimate cell are full grown, while those of the terminal 
cell have undergone a marked decrease in size. The nuclei are always 
present but are less than half of their former size. Not only is the 
nucleus as a whole reduced but also the densely staining body within it. 
Whether the dense spherical mass within the nucleus is a true nucleole 
or contains chromatin as well, I have been unable to determine. There 
is some evidence of a delicate chromatin network running through the 
nuclear cavity as figured by Blackman (4) in another rust, but these 
vegetative nuclei are minute and it is difficult to determine the details. 
The penultimate cell now regularly pushes out one or more slender 
hyphae just below the septum. This is seen in Plate 2, D, where a small 
budlike branch is just pushing out, and in A, just below the haustorium 
mother cell at 6, where the branch has already attained some length. 
The formation of a haustorium, then, does not stop the growth of a hypha 
permanently, for growth is practically always continued by means of 
these side branches. 
This haustorium-producing cell has a broad surface contact with the 
host cell, and its contents have become very dense (PI. 2, B). The 
branches below it in this case are slower than usual in forming but are 
beginning to push out. When the fungus is ready to enter, a minute 
pore is formed in the wall of the fungus cell and in the host cell wall 
next to it, probably by means of enzyms secreted by the fungus. The 
pore is ultramicroscopic in size; at least I have never seen it. Occa¬ 
sionally, however, at a later stage when the haustorium is formed, one 
finds a small circular red-stained spot on the host cell wall surrounding 
the point of entrance of the fungus. This suggests that the wall here 
