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AMERICAN JOURNAL OF BOTANY 
[Vol. 10, 
and then poured into a fresh “well” made into the potato, was followed, 
positive results were usually obtained. These results suggested strongly 
that an enzym was secreted which immediately began to macerate the host 
tissues in advance of the growth of the fungus. That dissolution of the 
middle lamellae takes place in advance of the mycelium is further borne 
out by the fact that a sterile zone of uncertain width is always present 
between the healthy tissue and the tips of the hyphae. In view of the 
fact, first, that an enzym seems to play such an important r 61 e in the decay 
of sweet potatoes after infection once takes place, and, second, that the 
original infection is difficult to obtain without first growing the fungus 
for one or two days on a nutrient medium in which the enzym is secreted, 
the writers proposed to make a detailed study of the part played by the 
enzym pectinase in infection. 
It should be pointed out here that, while the experiments dealing with 
this phase of the subject were made with either R. nigricans Ehrb. or 
R. tritici Saito, several other species also were found capable of decaying 
sweet potatoes (20) as well as a number of other vegetables and some 
fruits. All the different species were found to secrete pectinase (18) and 
to bring about a maceration of the host tissue in a similar way to that 
caused by R. tritici and R. nigricans. It is therefore believed that the role 
played by the enzym secreted by these two species is typical of that played 
by the other species of the genus. 
Pathological Histology 
Sweet potatoes decayed by Rhizopus are at first rendered very soft and 
stringy, water often dripping out of the potato when the skin is broken 
open. At the outset the color of the tissue is not changed, but later it 
turns a cinnamon to chocolate brown. On the escape of moisture the cells 
collapse, the potato dries up, and the whole mass finally becomes hard and 
mummified. Observed in this stage, it is often classed as dry rot. 
That the tissue is killed in advance of the fungous threads has been 
demonstrated by different methods. Attention has been called to the fact 
that plate plantings made from decayed tissue adjacent to healthy tissue 
were usually sterile. “Action in advance” was likewise demonstrated by 
a microscopic examination of stained sections made through the healthy 
and the adjacent decayed tissue in which the fungous threads were colored 
blue by Pianese’s stain. The results showed that maceration of the tissue 
had taken place several cells beyond the most advanced hyphae. Further¬ 
more, similar sections stained in methylene blue demonstrated the presence 
of the middle lamella connecting the cell walls of healthy tissue, while in 
the decayed tissue it had completely disappeared. 
If bits of decayed tissue floating in water are examined microscopically, 
the cells, although themselves remaining intact, will be seen to be separated 
from each other along the plane of the middle lamella. So far as the 
