250 
AMERICAN JOURNAL OF BOTANY 
[Vol. 10; 
are unable to penetrate even from within. If, however, some of the 
outer cells are ruptured, the fungus grows out and forms sporangia on 
the surface. Likewise, if sections of sweet potatoes cut to include some of 
the skin are immersed in an extract of the fungous hyphae or in a solution 
on which the fungus has grown, all the tissues except the cork are macerated 
and separate readily from it. The skin, therefore, forms an effective 
barrier to the penetration of Rhizopus. 
Small dead rootlets were proved to be the point of entrance of the fungus 
in a small percentage of the trials made. These experiments were conducted 
by the use of the method just described. A glass ring was sealed over an 
old dead rootlet, and a 48-hour-old culture of the organism grown in about 
2 cc. of sweet-potato decoction was poured into the cell, which was then 
covered with a cover slip. Out of a large number of such trials only about 
25 percent of the potatoes thus inoculated became infected, while 100 
percent of the controls inoculated by the well method decayed. It may be 
concluded from these results that the rootlets do not form as effective a 
barrier to the penetration of the fungus as the unbroken skin, although it 
was shown by Weimer and Harter (36) that a somewhat incomplete cork 
layer is laid down beneath the rootlets if the proper conditions of tempera¬ 
ture and humidity are maintained. It was also shown that if the solution 
on which the fungus had grown was poured into a glass cell sealed over a 
dead rootlet, more or less softening of the tissue took place in some cases. 
In other words, the dead rootlets formed a point of entrance which was 
not in all cases effectively protected by a cork layer. A number of sweet 
potatoes from the storage house, which had some wounds made during 
digging and storage but no apparently fresh ones, after being held under 
running water to remove some of the dirt, were immersed in a sweet-potato 
decoction on which the fungus had grown for 48 hours. After about 24 
hours in this solution, there was softening at the end where the potato was 
separated from the stem, in bruises and wounds made during digging and 
handling, and at certain places where small rootlets had died. These 
results show that in practically every potato certain wounds are present 
through which the enzym can enter; i.e., the skin which forms the only 
barrier to the entrance of the fungus is ruptured. 
A study was made of the extent of wounding necessary to permit infec¬ 
tion when the fungus was grown for one or two days on sweet-potato decoc¬ 
tion and the decoction and mycelium were used as an inoculum. Different 
types of wounds were tried. When such a growth was poured into a “well ” 
made by means of a cork-borer, infection usually resulted. On the other 
hand, only about 50 percent of the attempts to inoculate sweet potatoes 
through a small scratch just sufficient to rupture the skin were successful. 
When the skin was punctured once with a needle, the percentage of infection 
was even less (35 percent). These results show that a very small wound 
is sufficient to permit infection if the enzym is present. No infection 
