i68 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
Methods of Experimentation 
Two species of Rhizopus, R. tritici and R. nigricans , were employed. 
These two species were selected because among the parasitic species they 
represent the two extremes in the amount of pectinase produced. Rhizopus 
tritici , under the conditions of the writers’ previous experiments, secreted 
a very active macerating enzym while that secreted by R. nigricans was 
much less active. 
The spores were obtained in the following manner: Several 2-liter 
flasks containing about 500 cc. of sweet-potato decoction were inoculated 
with the organisms under investigation. At the end of ten days or two 
weeks spores were abundantly produced. The fungous felt was then care¬ 
fully lifted from the flasks, the lower side of the felt was held under the tap 
for a few moments to wash away the decoction, and then the felt was 
floated, the upper side down, in a vessel of distilled water. By gently 
agitating the fungous mat a considerable quantity though not all of the 
spores were removed. The spore suspension was then filtered through a 
good grade of muslin, about 27 threads to the centimeter, to remove any 
fungous threads that might have broken off during the process of removing 
the spores. At this stage a microscopic examination of the suspension was 
made, and if any bits of mycelium were found in the solution it was either 
filtered again or discarded. When the suspension was free of fungous 
debris the spores were caught on a number 2 chemically prepared Whatman 
filter paper. The spores were then washed from the filter paper with ace¬ 
tone. They were exposed to the acetone for 10 minutes, then caught on 
a tarred filter paper, and finally treated with ether by pouring the latter 
on to them on the filter paper. The filter paper was then removed from 
the funnel, straightened out, and stored away in any suitable container 
until required for use. Tests have shown that spores so treated will not 
germinate. 
After the dry weight of the spores was obtained, their macerating action 
on raw sweet-potato discs was determined by immersing the latter in a water 
suspension of the spores. At this point no attempt was made to separate 
the spores from the paper, the latter being included in the system. The 
amount of water used was determined by the weight of the spores. Ten 
cubic centimeters of water were used for every 0.10 gram of spores. Since 
it was impossible by the methods employed to obtain a definite weight of 
spores, the raw discs were exposed to a spore suspension of fairly uniform 
density by this adjustment of the relationship of the water and spores. 
Experimental Data 
Several experiments were conducted with the spores of both Rhizopus 
tritici and R. nigricans. In all cases except a few of the preliminary ones, 
the actual weight of the spores was determined. In every case one control 
