386 
Journal of Agricultural Research 
Vol. V, No. 9 
they gave identical results when grown on media containing cellulose, 
from various sources, or calcium pectinate. They were able in certain 
cases to hydrolyze the cellulose, but showed no dissolving action on calcium 
pectinate. 
In order to determine the difference between a soft-rot and a firm-rot 
caused by fungi which physiologically were acting alike in culture, apples 
rotting from P. expansum were examined. A smear of the rotted tissue 
revealed the fact that the host cells were entirely separated from one 
another, but that the walls were apparently intact. A few very small 
hyphae could be seen, seeming to be entirely intercellular. Further ex¬ 
amination of prepared slides of material, taken both from the oldest 
portion of a spot 3 inches in diameter and from the edge of the rotting 
spot, confirmed the above observations. The middle lamella was com¬ 
pletely split out between all of the cells in the rotted area, and the cel¬ 
lulose walls were entirely intact. The few very small hyphae that were 
found were intercellular (PI. XXXIX, fig. 4). So far as could be seen, 
the two fungi, 5 . cinerea and P. expansum , act in exactly the same way 
on the host tissue. The reason for one causing a firm-rot and the other 
a soft-rot is not, then, due to any differences in physiological action, but 
appears to be merely mechanical, due to the fact that 5 . cinerea com¬ 
pletely fills the intercellular space produced by the collapse of the cells 
(PI. XXXIX, fig. 5), with very large hyphae, while P. expansum pro¬ 
duces few small hyphae, which give little support to the host tissues, and, 
as a consequence, they collapse as the rot proceeds (PI. XXXIX, fig. 4). 
The complete solution of the middle lamella in tissue rotted by 
P. expansum would seem to indicate the presence of a middle-lamella¬ 
dissolving enzym. To test this, squares of very fine-grained filter paper 
were laid on blocks of apple and small portions of flesh from the edge 
of the rotting spot were laid on the filter papers. All precautions were 
observed, in order to keep the materials sterile. It was thought that 
if a pectinase were present it would filter through the paper and cause 
a soft-rot of the fruit. The papers bearing the rotted flesh were removed 
after 3 % hours. In four cases out of seven, infection took place 
through the filter paper and the normal soft-rot followed, while in the 
three other cases the blocks became soft and translucent at the end of 
two days, but showed no signs of infection. A microscopic examination 
showed the cells to be separated from one another, owing to the com¬ 
plete solution of the middle lamella. The checks remained firm. A 
small portion of the tissue, which rotted in the absence of hyphae, when 
transferred to the checks caused them to rot rapidly. This and the fact 
that in the typical rot spots the middle lamella is completely dissolved 
in the presence of very few hyphae would indicate that P. expansum 
secretes a very active middle-lamella-dissolving enzym, pectinase. 
