May, 1923] HARTER^ AND WEIMER — PECTINASE AND INFECTION 
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resulted when similar experiments were conducted using spores and hyphae 
in water and in the decoction instead of the one- and two-day-old cultures. 
Weimer and Harter (36) showed that cork is formed over wounds when 
the proper conditions of temperature and humidity are maintained. This 
wound cork was likewise found to exercise some resistance to infection by 
Rhizopus, but not complete protection against invasion. Numerous obser¬ 
vations of sweet potatoes in storage houses led to the general conclusion 
that infection there takes place at the ends of the potato more frequently 
than elsewhere, which would seem to indicate that what wound cork is 
formed there, together with the latex congealed over the surface, is not a 
complete protection against infection. Experiments in which 24-hour-old 
cultures of the fungus grown on sweet-potato decoction were poured into 
glass cells sealed over the ends of potatoes which had been in storage for 
some weeks showed that infection could take place through the ends. 
The results thus far show that infection takes place only through wounds. 
Furthermore, it is evident from the results that the wound need not be 
large, a mere needle prick being sufficient if the enzym is present. 
Saprophytic Start 
When sweet potatoes are dug, each one is wounded where it is broken 
from the vine. Furthermore, the skin of the potato is likely to be ruptured 
to some extent in the ordinary farm operations of digging, handling, and 
storing. The results of inoculation experiments with Rhizopus tritici on 
sweet potatoes and other crops show that it seldom if ever infects except 
through a wound of some sort. However, inoculation experiments have 
conclusively demonstrated that a wound alone is not sufficient to insure 
infection. Hundreds of attempts to infect sweet potatoes by smearing 
spores alone, and spores and hyphae dry, on the surface of a fresh wound 
have for the most part been unsuccessful, even when the potatoes thus 
inoculated were subjected to the temperature best suited to the growth of 
the organism and to a relatively high humidity such as that obtained by 
confining them in a moist chamber with wet filter paper on the bottom. 
Furthermore, if water in which spores and hyphae are suspended is 
poured into a “well” made into the potato by means of a cork-borer and 
covered with a cover slip to prevent evaporation, consistent infection does 
not take place. Likewise, if sweet-potato decoction is used instead of 
water, only a small percentage of infections results, in spite of the fact that 
the spores in both cases germinate and that those in the sweet-potato 
decoction form a considerable amount of fungous growth. Sweet potatoes 
absorb liquids quite readily, especially through a cut surface, and in all 
cases such as those just described the liquid was absorbed by the end of 24 
to 48 hours. Whether or not this is the reason why infection does not 
take place under the conditions of these experiments is not clear. However, 
results which throw some light on this question were obtained by experi- 
