May, 1923] HARTER AND WEIMER-PECTINASE AND INFECTION 
253 
Rhizopus grows readily on almost any kind of culture medium. A 
synthetic medium made somewhat acid to prevent the growth of bacteria 
is generally used in isolating the different species from dead or decayed 
material. Sweet potatoes which have been carefully washed have been 
dipped into a suspension of Rhizopus spores in synthetic agar. Infection 
and decay of potatoes so treated have always resulted. In these experi¬ 
ments no fresh wounds were made, infection in every case taking place 
through old wounds or at points where there were dead rootlets. A sweet 
potato completely covered with a coating of agar is under an abnormal 
condition, since respiration is largely cut off. Oxygen starvation is in the 
end inevitable, and the resistance of the host is doubtless greatly reduced. 
Other experiments were conducted in which agar with spores suspended in 
it was placed on a fresh wound. Decay took place as in the former case 
but not in so short a time, since infection occurred at only a single point. 
These and other experiments which will not be detailed show that if Rhizopus 
is given a saprophytic start, either by growing one or two days in a decoc¬ 
tion in agar on a wounded surface of the host, or in dead cells on the host, 
infection will almost invariably take place. 
The Relation of Certain Other Factors to Infection 
It is obvious that a temperature that will permit of the growth of the 
fungus must be maintained. The optimum temperature varies with the 
different species, as shown by Harter, Weimer, and Lauritzen (20). It is 
generally assumed that relatively high humidity is required for infection, 
but there is some evidence to show that infection will take place when the 
humidity of the air is comparatively low. Also, infection frequently fails 
when the humidity is relatively high. In fact, it is well known that sweet 
potatoes will become infected in the storage house under what might be 
regarded as fairly dry conditions. On the other hand, sweet potatoes, 
after being immersed in a spore suspension, often fail to decay when confined 
in a moist chamber lined with wet filter paper. Differences in the method 
of treatment previous to immersion in the spore suspension have given 
quite opposite results. If, for instance, sweet potatoes are cut in halves 
with a knife, dipped in a spore suspension, and then confined in a moist 
chamber, infection usually does not take place. If, on the other hand, the 
potatoes are struck against a blunt edge so as to make a wound to 
inch deep, and are then dipped in a spore suspension, infection almost 
invariably results. In the former case, the surface of the potato probably 
dries off by the absorption of the water before the spores can germinate 
and infect. In the latter case, on the other hand, the mutilated cells and 
the cell sap form a substratum which retains sufficient moisture to permit 
the germination of the spores and to provide for the subsequent growth of 
the mycelium. 
Reference has already been made to some experiments in which the 
