252 
AMERICAN JOURNAL OF BOTANY 
[Vol. io. 
merits in which a strip of cheesecloth with one end in a beaker of water and 
the other in the wound over the spores kept the spores constantly wet. 
Germination took place readily under these conditions, but infection did 
not occur. Likewise, when cut potatoes were immersed in a spore suspen¬ 
sion and then confined in a moist chamber, only a very low percentage of 
infection was obtained. As a matter of fact, the total percentage of decay 
by the use of this method was no greater than that of the controls which 
were not inoculated. 
The results of the investigation so far show that sweet potatoes are 
difficult to infect by the usual laboratory methods. On the other hand, 
potatoes wounded during the winter and kept in a commercial storage 
house frequently decay. The interesting question in this connection is 
why it is so difficult to infect sweet potatoes inoculated by the usual labora¬ 
tory methods while those freshly wounded and held in a storage house 
decay so readily. 
Keen (24) stated in respect to the decay of sweet potatoes caused by 
Rhizopus nigricans that the organism must first have a saprophytic start 
in order to become a parasite. He found that, if the spores were germinated 
and allowed to grow for a short time in orange juice, and were then trans¬ 
ferred to slices of sweet potatoes, decay would take place. Brooks (3) was 
likewise unable to infect lettuce with Botrytis cinerea by the use of spores 
alone, but if young mycelia were transferred to drops of grape extract, 
infection resulted. Neither of these investigators attempted to explain the 
principle underlying the “saprophytic start,” but it would seem that both 
are cases in which an enzym played an important role. 
So far as sweet potatoes are concerned, infection probably rarely takes 
place by the entrance of the hyphae directly into the healthy tissue, either 
wounded or unwounded. Many experiments, some of which have already 
been referred to, have shown that, even when potatoes are cut in two and 
dipped in a suspension of Rhizopus spores, infection, if it occurs at all, 
begins not on the cut surface but at some point at the edge of the cut 
where there is a bruise or dead tissue which serves to give the organism a 
saprophytic start. Such a “saprophytic start” is likewise furnished when 
the fungus is grown for one or two days in sweet-potato decoction. As 
already pointed out, when this method is followed, the decoction and 
fungous growth being confined in a “well” made into the potato, infection 
is practically assured. That this method is not the only one that enables 
the fungus to infect is evident from the following experiments. A number 
of potatoes were cut in two, and the cut surface of one half was held over 
a Bunsen burner until slightly charred. Treatment of this sort killed the 
tissue for several cells beneath the surface. All the halves of the sound 
and burned surfaces were smeared with spores of Rhizopus tritici . In 20 
hours the fungus was growing on the surfaces of the burned potatoes, and 
at the end of 2 days these potatoes were about three fourths decayed. 
The control halves, whose surfaces were not charred, remained sound. 
