Jan. 20,1923 
Cytological Study of Infection 
147 
where large numbers of competing mycelia are crowded in a leaf, the 
pustules formed are minute and produce few spores. The available food 
supply may be the main limiting factor. When, on the other hand, the 
mycelia are few and scattered, each attains to its full development and 
produces many spores. Moreover, in inoculations made by placing a 
single spore on a leaf of Early Baart, it has been noted that after the pus¬ 
tule has developed the mycelium spreads radially. Its extent can be 
determined by a slight discoloration of the leaf. Several days after the 
formation of the primary pustule, a circle (or ellipse, rather, for the 
growth is more rapid in the longitudinal direction) of secondary pustules 
forms around the margin of this spot. Given a free chance to develop, the 
damage done and the spore output resulting from the entrance of a single 
fungus into the host may be considerable. 
If, then, many spores are used in inoculation and only one in five 
produces infection the damage to the host and the resulting spore output 
of the fungus is certainly more than one-fifth of what it would be if all 
had entered. When relatively few scattered spores are placed on the 
plant and only one in five produces a mycelium, the results would be 
approximately one-fifth what they would be if all had entered. The 
latter would be usual under field conditions; and if later it should be 
proved that the stomata of Kanred behave in the same way in the field 
as they do in the greenhouse, this characteristic would have some 
economic importance. 
By becoming thoroughly familiar with the habits of the fungus in a 
congenial host, some light has been thrown on its history in an immune 
host. By learning to recognize the very regular and characteristic 
transformation through which a hypha goes before a haustorium is formed, 
and the^ appearance of the haustorium mother cell during and after the 
production of the haustorium, one learns to recognize the attempts of the 
fungus to form haustoria in an immune host and to see, in defeated 
attempts, the history written there. 
The fungus follows a normal course of development up to the time when 
the parasite enters the host cell. When, in Kanred, there is evidence of 
at least the beginning of the formation of a haustorium, and there has 
been actual contact between the cytoplasm of host and fungus, both the 
host cell and the fungus near it die. But a hypha can skim the surface of 
a cell, or even wrap itself around it, and, so long as no haustorium is 
initiated, host and fungus are unharmed. Stakman (27, p. 196), as 
already mentioned, also noted that a hypha in an immune host could 
pass by cells without harming them. 
When the reaction is slower after the entrance of the fungus into a host 
cell and haustoria succeed in growing to full size, more details of the pro¬ 
cess can be learned. The fact that the cytoplasmic envelope of the grow¬ 
ing haustorium in Kanred disappears rapidly points to the possibility 
that the fungus may be giving out some substance into the host cell. 
Certainly the host cell undergoes chemical changes, whether due to the 
mere physical presence of the haustorium or to some substance diffusing 
from it, and these changes result in the death of the host cell. Moreover, 
some substance, or maybe more than one, diffuses from the host cell 
into the haustorium; and this results either directly or indirectly in the 
death of the haustorium, the collapse of the cell producing it, and the 
plasmolysis or death of a portion of the penultimate cell below. This 
22329—23-2 
