Aug. i, 1921 
Studies in the Physiology of Parasitism 
615 
end point the personal factor naturally enters, but with experience it 
may be determined within relatively narrow limits. 
Immediately upon the removal of the fungous felt the solution was 
filtered through absorbent cotton to remove the remaining threads of 
hyphae. Cotton was used in preference to filter paper for reasons to be 
pointed out later. Except for special experiments to be discussed later 
the cultures were grown for only three days so that usually no, or only 
very few, spores were produced. Raw sweet-potato disks of the type 
mentioned above were immediately suspended in the filtered solution, 
incubated at the desired temperature, and records made of the progress of 
maceration. By this method of manipulation it is not unlikely that some 
spores, if produced, and some foreign organisms found their way into the 
solution. Pure cultures of the organisms were grown under sterile con¬ 
ditions, but as soon as the fungous felt was removed it as well as the solu¬ 
tion doubtless became contaminated by foreign organisms from the air. 
The extracts and the solutions were so powerful in pectinase that macera¬ 
tion of the tissues was complete before these organisms could influence 
the results appreciably. In this connection it may be stated that in no 
case have contaminating bacteria been found to exercise any macerating 
action on raw disks of sweet potatoes. In cases where the macerating 
action was prolonged beyond two or three hours the surface of the liquid 
was covered with toluol. In a number of trial experiments it was found 
that no foreign organisms could be isolated from solutions which had been 
well shaken with toluol. 
Other investigators in this field have carried out most of their experi¬ 
ments by the use of an extract of the fungous material. This is usually 
obtained by grinding the mass of hyphae, suspending it in water for a 
definite length of time, then filtering out the fungous debris. This 
method was not followed by the writers for reasons which will be made 
clear later. The ground mycelium was suspended in the water and formed 
a part of the system to which the disks were added. Several objections 
may be offered against the extraction of the hyphae. First, it allows 
foreign organisms, which doubtless are introduced during the process of 
manipulation, to multiply. Second, a considerably longer time is required 
to complete an experiment and, third, all the enzym may not be extracted 
and some of it may be removed by the filter paper. 
EXPERIMENTAL DATA 
PECTINASE FORMATION BY RHIZOPUS TRITICI 
At the outset of the storage-rot experiments it was sometimes difficult 
to obtain a culture of Rhizopus from rotted sweet potato, although there 
was an abundance of evidence that this fungus caused the decay. In 
order to avoid foreign organisms the plate plantings were made from 
partially decayed tissue adjacent to the healthy zone. Plates so made 
